Thin film transistor and method for manufacturing the same

ABSTRACT

An object is to provide a thin film transistor with small off current, large on current, and high field-effect mobility. A silicon nitride layer and a silicon oxide layer which is formed by oxidizing the silicon nitride layer are stacked as a gate insulating layer, and crystals grow from an interface of the silicon oxide layer of the gate insulating layer to form a microcrystalline semiconductor layer; thus, an inverted staggered thin film transistor is manufactured. Since crystals grow from the gate insulating layer, the thin film transistor can have a high crystallinity, large on current, and high field-effect mobility. In addition, a buffer layer is provided to reduce off current.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin film transistor and a method formanufacturing the thin film transistor, and a semiconductor device and adisplay device formed using the thin film transistor.

2. Description of the Related Art

In recent years, a thin film transistor including a thin semiconductorfilm (with a thickness of several nanometers to several hundreds ofnanometers, approximately) over a substrate having an insulating surface(e.g., a glass substrate) has been attracting attention. Thin filmtransistors are widely used for ICs (integrated circuits) and electronicdevices such as electro-optical devices. In particular, thin filmtransistors are urgently developed as switching elements of imagedisplay devices typified by liquid crystal display devices and the like.In an image display device such as a liquid crystal display device, athin film transistor including an amorphous semiconductor layer or athin film transistor including a polycrystalline semiconductor layer ismainly used as a switching element.

As a switching element of an image display device, a thin filmtransistor in which a microcrystalline semiconductor layer is used for achannel formation region is known in addition to a thin film transistorin which an amorphous semiconductor layer is used for a channelformation region and a thin film transistor in which a polycrystallinesemiconductor layer is used for a channel formation region (see PatentDocument 1).

Further, there is a method in which characteristics of a thin filmtransistor are improved by exposure of the thin film transistor to anatmosphere containing oxygen ions and oxygen active species generated byplasma discharge in a gas containing oxygen (see Patent Document 2).

[Reference] [Patent Document] [Patent Document 1] Japanese PublishedPatent Application No. 2009-044134

[Patent Document 2] Japanese Published Patent Application No. H6-177142

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to reduce offcurrent of a thin film transistor and to increase on current andfield-effect mobility.

Another object of one embodiment of the present invention is to increaseproductivity of a thin film transistor with small off current, large oncurrent, and high field-effect mobility.

One embodiment of the present invention is an inverted staggered thinfilm transistor in which a silicon nitride layer and a silicon oxidelayer formed by oxidizing a surface of the silicon nitride layer arestacked as a gate insulating layer and crystals grow from an interfaceof the silicon oxide layer of the gate insulating layer to form amicrocrystalline semiconductor layer.

Another embodiment of the present invention is a thin film transistorincluding a gate electrode, a gate insulating layer which covers thegate electrode, a semiconductor layer including a microcrystallinesemiconductor layer over the gate insulating layer, and a source regionand a drain region which are in contact with the semiconductor layer.The nitrogen concentration at an interface between the gate insulatinglayer and the microcrystalline semiconductor layer is higher than orequal to 5×10¹⁹ atoms/cm³ and lower than or equal to 1×10²² atoms/cm³.The nitrogen concentration in the microcrystalline semiconductor layerreaches the minimum value, which is higher than or equal to 1×10¹⁷atoms/cm³ and lower than or equal to 3×10¹⁹ atoms/cm³.

Another embodiment of the present invention is a thin film transistorincluding a gate electrode, a gate insulating layer which covers thegate electrode, a first semiconductor layer which is formed using amicrocrystalline semiconductor layer over the gate insulating layer, asecond semiconductor layer which includes a conical or pyramidal crystalregion and an amorphous semiconductor region over the firstsemiconductor layer, and a source region and a drain region which are incontact with the second semiconductor layer. The nitrogen concentrationat an interface between the gate insulating layer and the firstsemiconductor layer is higher than or equal to 5×10¹⁹ atoms/cm³ andlower than or equal to 1×10²² atoms/cm³. The nitrogen concentration inthe first semiconductor layer reaches the minimum value, which is higherthan or equal to 1×10¹⁷ atoms/cm³ and lower than or equal to 3×10¹⁹atoms/cm³. The nitrogen concentration in the second semiconductor layeris higher than or equal to 1×10¹⁹ atoms/cm³ and lower than or equal to1×10²¹ atoms/cm³. The oxygen concentration at an interface between thegate insulating layer and the first semiconductor layer is higher thanor equal to 1×10¹⁹ atoms/cm³ and lower than or equal to 1×10²²atoms/cm³. The oxygen concentration in the second semiconductor layer islower than or equal to 1×10¹⁸ atoms/cm³.

As a method for oxidation treatment of the silicon nitride layer whichis part of the gate insulating layer, there is a method in which thesilicon nitride layer is exposed to plasma generated in an atmospherecontaining oxygen. When the silicon nitride layer is exposed to plasmagenerated in an atmosphere containing oxygen, a treatment chamber wherethe silicon nitride layer is formed or another treatment chamber may beused. When the silicon nitride layer is exposed to plasma generated inan atmosphere containing oxygen, the substrate provided with the siliconnitride layer is not carried out of a vacuum apparatus and thusthroughput is high, which is preferable.

As another method for oxidation treatment of the silicon nitride layerwhich is part of the gate insulating layer, there is a method in whichthe silicon nitride layer is exposed to an atmosphere containing ozone.

As another method for oxidation treatment of the silicon nitride layerwhich is part of the gate insulating layer, there is a method in whichthe silicon nitride layer is soaked in ozone water or the like.

Alternatively, the silicon oxide layer may be formed over the siliconnitride layer by a CVD method.

The silicon oxide layer may have a thickness which enables the siliconoxide layer to cover a surface of the silicon nitride layer and whichdoes not take long time for etching the gate insulating layer which isperformed to expose the gate electrode layer (that is, time for etchingthe silicon oxide layer is approximately same as that for etching thesilicon nitride layer). The silicon oxide layer is preferably formed tohave a thickness of greater than or equal to 2 nm and less than 10 nm.

As the gate insulating layer, the silicon oxide layer is stacked overthe silicon nitride layer, so that the nitrogen concentration at aninterface between the gate insulating layer and the first semiconductorlayer formed using a microcrystalline semiconductor can be reduced, andcrystals grow from a surface of the gate insulating layer to form amicrocrystalline semiconductor layer as the first semiconductor layer.When the uppermost layer of the gate insulating layer is a silicon oxidelayer with a thickness of greater than or equal to 2 nm and less than 10nm, reduction in throughput in an etching process for exposing the gateelectrode can be prevented.

It is possible to reduce off current of a thin film transistor and toincrease on current and field-effect mobility. Further, a thin filmtransistor with small off current, large on current, and highfield-effect mobility can be manufactured with high productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B each illustrate a thin film transistor;

FIGS. 2A and 2B each illustrate a semiconductor layer included in a thinfilm transistor;

FIG. 3 illustrates a semiconductor layer included in a thin filmtransistor;

FIGS. 4A and 4B each illustrate a semiconductor layer included in a thinfilm transistor;

FIGS. 5A to 5D illustrate a method for manufacturing a thin filmtransistor;

FIGS. 6A to 6C illustrate a method for manufacturing a thin filmtransistor;

FIG. 7 illustrates an example of a plasma treatment apparatus;

FIG. 8 illustrates an example of a timing chart of a method formanufacturing a thin film transistor;

FIG. 9 illustrates concentrations of elements included in a gateinsulating layer, a first semiconductor layer, and a secondsemiconductor layer;

FIGS. 10A to 10C illustrate an example of a method for manufacturing athin film transistor;

FIGS. 11A and 11B illustrate an example of a method for manufacturing athin film transistor;

FIGS. 12A and 12B illustrate multi-tone masks;

FIG. 13 illustrates an example of a display panel;

FIGS. 14A and 14B illustrate an example of a display panel;

FIGS. 15A and 15B illustrate an example of a display panel;

FIGS. 16A to 16D each illustrate an electronic device;

FIGS. 17A to 17C illustrate an electronic device;

FIG. 18 illustrates a measurement result of XPS;

FIG. 19 illustrates concentrations of elements included in a gateinsulating layer, a first semiconductor layer, and a secondsemiconductor layer;

FIGS. 20A to 20C are cross-sectional STEM images described in Example 3;

FIGS. 21A to 21C illustrate simulation results of a crystallizationprocess of a microcrystalline semiconductor layer;

FIGS. 22A to 22C illustrate simulation results of a crystallizationprocess of a microcrystalline semiconductor layer;

FIGS. 23A to 23C illustrate simulation results of a crystallizationprocess of a microcrystalline semiconductor layer; and

FIGS. 24A to 24C illustrate simulation results of a crystallizationprocess of a microcrystalline semiconductor layer.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and examples of the present invention aredescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to the followingdescription and it is easily understood by those skilled in the art thatthe mode and details can be variously changed without departing from thescope and spirit of the present invention. Accordingly, the presentinvention should not be construed as being limited to the description ofthe embodiments and examples below. In describing structures of thepresent invention with reference to drawings, the same referencenumerals are used in common for the same portions in different drawings.Note that the same hatch pattern is applied to similar parts, and thesimilar parts are not especially denoted by reference numerals in somecases. In addition, an insulating layer is not illustrated in a top viewfor convenience in some cases. Note that the size, the thickness of alayer, or a region of each structure illustrated in drawings areexaggerated for simplicity in some cases. Therefore, the presentinvention is not necessarily limited to such scales illustrated in thedrawings.

Embodiment 1

FIGS. 1A and 1B each illustrate a cross-sectional view of a thin filmtransistor which is one embodiment of the present invention. A thin filmtransistor illustrated in FIG. 1A includes, over a substrate 400, a gateelectrode layer 402, a gate insulating layer 404 which covers the gateelectrode layer 402, a semiconductor layer in which a firstsemiconductor layer 406 and a second semiconductor layer 407 are stackedover the gate insulating layer 404, source and drain regions 410 whichare provided in contact with the semiconductor layer, and source anddrain electrode layers 412 which are provided in contact with the sourceand drain regions 410. The second semiconductor layer 407 is provided bystacking a mixed region 408 and a region 409 including an amorphoussemiconductor. The gate insulating layer 404 is provided by stacking asilicon nitride layer 404A and a silicon oxide layer 404B.

FIG. 1B is an enlarged view of a rectangular region 430 illustrated inFIG. 1A. Part 431 of a channel formation region, a back channel portion432, a depletion layer 433, and a bond region 434 are illustrated inFIG. 1B.

The part 431 of a channel formation region is overlapped with the gateelectrode layer 402 and formed on the gate insulating layer 404 side.The back channel portion 432 is formed in an exposed region of theregion 409 including an amorphous semiconductor between the source anddrain regions. The depletion layer 433 is formed in the vicinity of aportion of the region 409 including an amorphous semiconductor incontact with the drain region. The bond region 434 is formed in aportion where the region 409 including an amorphous semiconductor is incontact with the source or drain region.

One of features of the thin film transistor of this embodiment is thatthe gate insulating layer of the thin film transistor is provided bystacking the silicon nitride layer 404A and the silicon oxide layer404B.

The silicon nitride layer 404A is formed using silicon nitride orsilicon nitride oxide. The thickness of the silicon nitride layer 404Ais 50 nm or more, preferably 50 nm to 400 nm inclusive, more preferably150 nm to 300 nm inclusive. The silicon nitride layer 404A is providedbetween the substrate 400 and the first semiconductor layer 406, so thatimpurities (particularly an alkali metal ion or the like included in thesubstrate 400) from the substrate 400 can be prevented from being mixedinto the first semiconductor layer 406 which is formed in a later step;therefore, fluctuation in the threshold voltage of the thin filmtransistor can be reduced.

The silicon oxide layer 404B is formed using silicon oxide or siliconoxynitride. The silicon oxide layer 404B may have a thickness whichenables the silicon oxide layer 404B to retain a film shape to cover asurface of the silicon nitride layer 404A and which does not reducethroughput extremely due to increase in time for etching the gateinsulating layer 404 which is performed to expose the gate electrodelayer 402 to be described later (that is, a thickness with which thetime for etching the silicon oxide layer 404B is approximately the sameas that for etching the silicon nitride layer 404A). The thickness ofthe silicon oxide layer 404B is preferably greater than or equal to 2 nmand less than 10 nm.

In this specification, silicon oxynitride contains more oxygen thannitrogen, and in the case where measurements are conducted usingRutherford backscattering spectrometry (RBS) and hydrogen forwardscattering spectrometry (HFS), silicon oxynitride preferably containsoxygen, nitrogen, silicon, and hydrogen as composition ranging from 50atomic % to 70 atomic %, 0.5 atomic % to 15 atomic %, 25 atomic % to 35atomic %, and 0.1 atomic % to 10 atomic %, respectively. Silicon nitrideoxide contains more nitrogen than oxygen, and in the case wheremeasurements are conducted using RBS and HFS, silicon nitride oxidepreferably contains oxygen, nitrogen, silicon, and hydrogen ascomposition ranging from 5 atomic % to 30 atomic %, 20 atomic % to 55atomic %, 25 atomic % to 35 atomic %, and 10 atomic % to 30 atomic %,respectively. Note that percentages of nitrogen, oxygen, silicon, andhydrogen fall within the ranges given above, where the total number ofatoms contained in the silicon oxynitride or the silicon nitride oxideis defined as 100 atomic %.

Here, the nitrogen concentration and the oxygen concentration from thegate insulating layer 404 to the second semiconductor layer 407 aredescribed with reference to FIG. 9.

FIG. 9 shows results of analysis by secondary ion mass spectrometry of asample formed by stacking an insulating layer to be the gate insulatinglayer 404, a semiconductor layer to be the first semiconductor layer406, a semiconductor layer to be the second semiconductor layer 407, andan impurity semiconductor layer to be the source and drain regions 410in this order over the substrate 400. The horizontal axis represents thedepth from the surface of the sample. The vertical axis on the left siderepresents the concentrations of hydrogen, nitrogen, and oxygen. Thevertical axis on the right side represents the secondary ion intensityof silicon. A depressed portion at a depth of approximately 45 nm to 50nm in the secondary ion intensity of silicon is an interface between thesecond semiconductor layer 407 and the source and drain regions 410. Apeak portion at a depth of approximately 240 nm to 245 nm in thesecondary ion intensity of silicon is an interface between the gateinsulating layer 404 and the first semiconductor layer 406. Here, thethickness of the first semiconductor layer 406 is approximately 30 nm;accordingly, it is estimated that an interface between the firstsemiconductor layer 406 and the second semiconductor layer 407 is at adepth of approximately 210 nm to 215 nm.

According to FIG. 9, the nitrogen concentration sharply decreases at theinterface between the gate insulating layer 404 and the firstsemiconductor layer 406, and then gradually increases toward the secondsemiconductor layer 407.

Specifically, the nitrogen concentration reaches the minimum value inthe first semiconductor layer 406. Note that the nitrogen concentrationis substantially constant in the second semiconductor layer 407.

The oxygen concentration sharply increases at the interface between thegate insulating layer 404 and the first semiconductor layer 406 and thensharply decreases. After that, the oxygen concentration graduallydecreases toward the second semiconductor layer 407. Specifically, theoxygen concentration reaches the minimum value also in the firstsemiconductor layer 406. Note that the oxygen concentration issubstantially constant in the second semiconductor layer 407.

The nitrogen concentration at the interface between the gate insulatinglayer 404 and the first semiconductor layer 406 (the nitrogenconcentration before rapid decrease) is higher than or equal to 5×10¹⁹atoms/cm³ and lower than or equal to 1×10²² atoms/cm³. The minimum valueof the nitrogen concentration in the first semiconductor layer 406 ishigher than or equal to 1×10¹⁷ atoms/cm³ and lower than or equal to3×10¹⁹ atoms/cm³. The nitrogen concentration in the second semiconductorlayer 407 is higher than or equal to 1×10¹⁹ atoms/cm³ and lower than orequal to 1×10²¹ atoms/cm³.

The oxygen concentration at the interface between the gate insulatinglayer 404 and the first semiconductor layer 406 is higher than or equalto 1×10¹⁹ atoms/cm³ and lower than or equal to 1×10²² atoms/cm³. Theoxygen concentration in the second semiconductor layer 407 is lower thanor equal to 1×10¹⁸ atoms/cm³.

In the case where a microcrystalline semiconductor is formed as thefirst semiconductor layer 406, the crystal growth rate decreases whenthe nitrogen concentration is high and crystal growth does not proceedat an early stage of deposition of the microcrystalline semiconductor,resulting in formation of an amorphous semiconductor. However, nitrogenis contained in the first semiconductor layer 406 so that the minimumvalue of the nitrogen concentration is higher than or equal to 1×10¹⁷atoms/cm³ and lower than or equal to 3×10¹⁹ atoms/cm³, whereby thenitrogen concentration at the interface between the gate insulatinglayer 404 and the first semiconductor layer 406 can be reduced andcrystals of the first semiconductor layer 406 can grow from a surface ofthe gate insulating layer 404. Moreover, the oxygen concentration at theinterface between the gate insulating layer 404 and the firstsemiconductor layer 406 is set to higher than or equal to 1×10¹⁹atoms/cm³ and lower than or equal to 1×10²² atoms/cm³, whereby amicrocrystalline semiconductor layer having high crystallinity can beformed as the first semiconductor layer 406. Accordingly, on current andfield-effect mobility of the thin film transistor can be increased.

Further, the nitrogen concentration increases at the interface betweenthe first semiconductor layer 406 and the second semiconductor layer 407and the nitrogen concentration in the second semiconductor layer 407 ismade to be constant, whereby the mixed region 408 including amicrocrystalline semiconductor region and an amorphous semiconductorregion is formed and then the region 409 including a well-orderedamorphous semiconductor which has few defects and a steep tail of alevel at a band edge in the valence band is formed. As a result, in thethin film transistor, when voltage is applied to a source or drainelectrode, resistance between the gate insulating layer and the sourceand drain regions can be reduced and a back channel portion includes fewdefects, whereby off current of the thin film transistor can be reduced.

As the substrate 400, any of the following substrates can be used: analkali-free glass substrate formed using barium borosilicate glass,aluminoborosilicate glass, aluminosilicate glass, or the like by afusion method or a float method; a ceramic substrate; a plasticsubstrate having heat resistance enough to withstand a processtemperature of a process for manufacturing the thin film transistordisclosed in this embodiment; and the like. Alternatively, a metalsubstrate of a stainless steel alloy or the like with a surface providedwith an insulating layer may be used. That is, a substrate having aninsulating surface is used as the substrate 400. When the substrate 400is mother glass, the substrate may have any size of the first generation(e.g., 320 mm×400 mm) to the tenth generation (e.g., 2950 mm×3400 mm),and the like.

The gate electrode layer 402 may be formed using a conductive material.As an example of the conductive material, a metal material such asmolybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, or scandium or an alloy material containing any of thesematerials as a main component can be used. Alternatively, crystallinesilicon to which an impurity element imparting one conductivity type isadded may be used for the gate electrode layer 402. Note that the gateelectrode layer 402 may have a single layer structure or a structure inwhich a plurality of layers are stacked. For example, a two-layerstructure in which a titanium layer or a molybdenum layer is stackedover an aluminum layer or a copper layer may be used. Alternatively, athree-layer structure in which an aluminum layer or a copper layer issandwiched between titanium layers or molybdenum layers may be used.Further, a titanium nitride layer may be used instead of a titaniumlayer. Note that the conductive material is not limited to the abovedescribed materials.

Next, the first semiconductor layer 406 and the second semiconductorlayer 407 are described with reference to FIGS. 2A and 2B, FIG. 3, andFIGS. 4A and 4B. FIGS. 2A and 2B, FIG. 3, and FIGS. 4A and 4B eachillustrate an enlarged view of a region between the gate insulatinglayer 404 and the source and drain regions 410 in FIGS. 1A and 1B.

A microcrystalline semiconductor included in the first semiconductorlayer 406 has crystalline structures (including single crystal andpolycrystal). A microcrystalline semiconductor is a semiconductor havinga third state that is stable in terms of free energy and a crystallinesemiconductor having short-range order and lattice distortion, in whichcolumnar or needle-like crystals having a crystal grain size of 2 nm to200 nm inclusive, preferably 10 nm to 80 nm inclusive, more preferably20 nm to 50 nm inclusive, grow in a normal direction with respect to asubstrate surface. Therefore, there is a case where crystal grainboundaries are formed at the interface of columnar or needle-likecrystals. As a typical example of a microcrystalline semiconductor,microcrystalline silicon, microcrystalline silicon germanium,microcrystalline germanium, or the like can be given. Note that animpurity element serving as a donor may be included in themicrocrystalline semiconductor. Typical examples of the impurity elementserving as a donor include phosphorus, arsenic, and antimony which areGroup 15 elements of the periodic table, and the like. An impurityelement serving as an acceptor may be included in the microcrystallinesemiconductor in order to control the threshold value of the thin filmtransistor. Boron is given as an example of the impurity element servingas an acceptor.

Microcrystalline silicon which is a typical example of themicrocrystalline semiconductor has a peak of Raman spectrum which isshifted to a lower wave number side than 520 cm⁻¹ that represents singlecrystal silicon. That is, microcrystalline silicon has the peak of theRaman spectrum between 520 cm⁻¹ that represents single crystal siliconand 480 cm⁻¹ that represents amorphous silicon. The microcrystallinesemiconductor may contain hydrogen or halogen of at least 1 atomic % toterminate a dangling bond. Moreover, a rare gas element such as helium,argon, krypton, or neon may be included in the source gas to furtherpromote lattice distortion, so that stability of the structure ofmicrocrystals is enhanced and a microcrystalline semiconductor withfavorable characteristics can be obtained. Such a microcrystallinesemiconductor is disclosed in, for example, U.S. Pat. No. 4,409,134.

The thickness of the first semiconductor layer 406 is preferably greaterthan or equal to 3 nm and less than or equal to 100 nm, more preferablygreater than or equal to 5 nm and less than or equal to 50 nm. In thecase where the first semiconductor layer 406 is too thin, on current ofthe thin film transistor is reduced. In the case where the firstsemiconductor layer 406 is too thick, off current of the thin filmtransistor is increased when the thin film transistor operates at a hightemperature. Therefore, the thickness of the first semiconductor layer406 is set to greater than or equal to 3 nm and less than or equal to100 nm, preferably greater than or equal to 5 nm and less than or equalto 50 nm, so that on current and off current of the thin film transistorcan be adjusted to appropriate values.

Although the first semiconductor layer 406 illustrated in FIGS. 1A and1B, FIGS. 2A and 2B, FIG. 3, and FIGS. 4A and 4B has a layeredstructure, microcrystalline semiconductor particles may be dispersedover the gate insulating layer 404. In this case, the mixed region 408is made to be in contact with the microcrystalline semiconductorparticles and the gate insulating layer 404.

Each of the microcrystalline semiconductor particles can existindependently when the size of the microcrystalline semiconductorparticle is set to greater than or equal to 1 nm and less than or equalto 30 nm and the number of the particles per unit area is set to lessthan 1×10¹³/cm², preferably less than 1×10¹⁰/cm².

As described above, the second semiconductor layer 407 includesnitrogen. The concentration of nitrogen included in the secondsemiconductor layer 407 is set to higher than or equal to 1×10¹⁹atoms/cm³ and lower than or equal to 1×10²¹ atoms/cm³, preferably higherthan or equal to 1×10²⁰ atoms/cm³ and lower than or equal to 1×10²¹atoms/cm³, more preferably higher than or equal to 2×10²⁰ atoms/cm³ andlower than or equal to 1×10²¹ atoms/cm³.

As illustrated in FIG. 2A, the mixed region 408 includes amicrocrystalline semiconductor region 427A which grows into a projectingshape from a surface of the first semiconductor layer 406 and anamorphous semiconductor region 427B which fills a space between themicrocrystalline semiconductor regions 427A. The region 409 including anamorphous semiconductor is formed using the semiconductor materialsimilar to that of the amorphous semiconductor region 427B.

The thickness of the second semiconductor layer 407 is preferablygreater than or equal to 50 nm and less than or equal to 350 nm, morepreferably greater than or equal to 120 nm and less than or equal to 250nm.

The microcrystalline semiconductor region 427A is a microcrystallinesemiconductor having a projecting (conical or pyramidal) shape whose endis narrowed from the gate insulating layer 404 toward the region 409including an amorphous semiconductor. Note that the microcrystallinesemiconductor region 427A may be a microcrystalline semiconductor whichhas a projecting (inverted conical or inverted pyramidal) shape having awidth increased from the gate insulating layer 404 toward the region 409including an amorphous semiconductor.

Note that the amorphous semiconductor region 427B included in the mixedregion 408 may contain a semiconductor crystal grain having a grain sizeof 1 nm to 10 nm inclusive, preferably 1 nm to 5 nm inclusive.

Alternatively, as illustrated in FIG. 2B, the mixed region 408 includesa microcrystalline semiconductor region 427C and the microcrystallinesemiconductor region 427A which are successively formed, in some cases.The microcrystalline semiconductor region 427C is deposited with auniform thickness over the first semiconductor layer 406. Themicrocrystalline semiconductor region 427A has a projecting (conical orpyramidal) shape whose end is narrowed from the gate insulating layer404 toward the region 409 including an amorphous semiconductor.

Note that, in FIGS. 2A and 2B, the amorphous semiconductor region 427Bincluded in the mixed region 408 is a semiconductor, the quality ofwhich is substantially the same as the quality of the region 409including an amorphous semiconductor.

According to the above, an interface between a region formed using amicrocrystalline semiconductor and a region formed using an amorphoussemiconductor may correspond to the interface between themicrocrystalline semiconductor region 427A and the amorphoussemiconductor region 427B in the mixed region 408. Therefore, theinterface between the microcrystalline semiconductor region and theamorphous semiconductor region can be described as uneven or zigzag in across-sectional view.

In the mixed region 408, in the case where the microcrystallinesemiconductor region 427A is a semiconductor crystal grain having aprojecting (conical or pyramidal) shape whose end is narrowed from thegate insulating layer 404 toward the region 409 including an amorphoussemiconductor, the proportion of the microcrystalline semiconductor inthe vicinity of the first semiconductor layer 406 is higher than that inthe vicinity of the region 409 including an amorphous semiconductor. Themicrocrystalline semiconductor region 427A grows in a thicknessdirection from the surface of the first semiconductor layer 406.However, crystal growth of the microcrystalline semiconductor region427A is suppressed by adding a gas containing nitrogen to the source gasor by adding a gas containing nitrogen to the source gas and reducingthe flow rate of hydrogen to silane from that under the condition forforming the first semiconductor layer 406, the semiconductor crystalgrain becomes a conical or pyramidal shape, and the amorphoussemiconductor is gradually deposited. This is because the solidsolubility of nitrogen in the microcrystalline semiconductor region islower than the solid solubility of nitrogen in the amorphoussemiconductor region.

The total thickness of the first semiconductor layer 406 and the mixedregion 408, that is, the distance from the interface between the gateinsulating layer 404 and the silicon oxide layer 404B to the tip of theprojection (projecting portion) of the mixed region 408, is set togreater than or equal to 3 nm and less than or equal to 410 nm,preferably greater than or equal to 20 nm and less than or equal to 100nm. The total thickness of the first semiconductor layer 406 and themixed region 408 is set to greater than or equal to 3 nm and less thanor equal to 410 nm, preferably greater than or equal to 20 nm and lessthan or equal to 100 nm, so that off current of the thin film transistorcan be reduced.

As described above, the region 409 including an amorphous semiconductoris a semiconductor, the quality of which is substantially the same asthe quality of the amorphous semiconductor region 4278, and containsnitrogen. Further, the region 409 including an amorphous semiconductorcontains a semiconductor crystal grain having a grain size of 1 nm to 10nm inclusive, preferably 1 nm to 5 nm inclusive, in some cases. Here,the region 409 including an amorphous semiconductor is a semiconductorlayer having lower energy at an Urbach edge and a smaller amount of theabsorption spectrum of defects, measured by a constant photocurrentmethod (CPM) or photoluminescence spectroscopy, as compared to aconventional amorphous semiconductor. That is, as compared to theconventional amorphous semiconductor, the region 409 including anamorphous semiconductor is a well-ordered semiconductor which has fewerdefects and a steep tail of a level at a band edge in the valence band.Since the region 409 including an amorphous semiconductor has a steeptail of a level at a band edge in the valence band, the band gap getswider and tunneling current does not easily flow. Therefore, byproviding the region 409 including an amorphous semiconductor on theback channel side, off current of the thin film transistor can bereduced. In addition, by providing the region 409 including an amorphoussemiconductor, on current and field-effect mobility can be increased.

Further, a peak region of a spectrum obtained by performinglow-temperature photoluminescence spectroscopy on the region 409including an amorphous semiconductor is 1.31 eV to 1.39 eV inclusive.Note that a peak region of a spectrum obtained by performinglow-temperature photoluminescence spectroscopy on a microcrystallinesemiconductor layer such as a microcrystalline silicon layer is 0.98 eVto 1.02 eV inclusive. Accordingly, the region 409 including an amorphoussemiconductor is different from a microcrystalline semiconductor layer.

Note that an amorphous semiconductor included in the region 409including an amorphous semiconductor is amorphous silicon, for example.

Note that nitrogen included in the mixed region 408 and the region 409including an amorphous semiconductor preferably exists as an NH group oran NH₂ group, for example. This is because dangling bonds of asemiconductor atom are cross-linked with a nitrogen atom or an NH groupor terminated with an NH₂ group, and thus carriers flow easily.

Alternatively, as illustrated in FIG. 3, the mixed region 408 mayentirely fill a space between the first semiconductor layer 406 and thesource and drain regions 410. It is preferable that the structureillustrated in FIG. 3 have the proportion of the microcrystallinesemiconductor region 427A in the mixed region 408 illustrated in FIG. 3lower than that illustrated in FIGS. 2A and 2B. Further, the proportionof the microcrystalline semiconductor region 427A in the mixed region408 is preferably low in a region between the source and drain regions,that is, a region where carriers flow. As a result, off current of thethin film transistor can be reduced. In addition, in the mixed region408, it is possible to reduce resistance in a vertical direction (athickness direction), that is, resistance between the semiconductorlayer and the source and drain regions, when the thin film transistor isin an on state and voltage is applied between the source and drainelectrode layers 412, and thus on current and field-effect mobility ofthe thin film transistor can be increased.

Note that the mixed region 408 illustrated in FIG. 3 may include themicrocrystalline semiconductor region 427C as illustrated in FIG. 2B.

Further, a conventional amorphous semiconductor layer 429D may beprovided between the region 409 including an amorphous semiconductor andthe source and drain regions 410 as illustrated in FIG. 4A.Alternatively, the conventional amorphous semiconductor layer 429D maybe provided between the mixed region 408 and the source and drainregions 410 as illustrated in FIG. 4B. The structures illustrated inFIGS. 4A and 413 enable off current of the thin film transistor to bereduced.

Note that the mixed region 408 illustrated in FIGS. 4A and 4B mayinclude the microcrystalline semiconductor region 427C as illustrated inFIG. 2B.

Since the mixed region 408 includes the microcrystalline semiconductorregion 427A having a conical or pyramidal shape, it is possible toreduce resistance in a vertical direction (a thickness direction), thatis, resistance of the first semiconductor layer 406, the mixed region408, and the region 409 including an amorphous semiconductor, when thethin film transistor is in an on state and voltage is applied betweenthe source and drain electrodes.

As described above, the second semiconductor layer 407 contains nitrogen(e.g., an NH group or an NH₂ group) in some cases. This is becausedefects are reduced when nitrogen (e.g., an NH group or an NH₂ group) isbonded to dangling bonds of silicon atoms at the interface between aplurality of the microcrystalline semiconductor regions included in themicrocrystalline semiconductor region 427A, the interface between themicrocrystalline semiconductor region 427A and the amorphoussemiconductor region 427B, or the interface between the firstsemiconductor layer 406 and the amorphous semiconductor region 427B.Accordingly, the nitrogen concentration of the second semiconductorlayer 407 is set to higher than or equal to 1×10¹⁹ atoms/cm³ and lowerthan or equal to 1×10²¹ atoms/cm³, preferably higher than or equal to1×10²⁰ atoms/cm³ and lower than or equal to 1×10²¹ atoms/cm³, and thusthe dangling bonds of silicon atoms can be easily cross-linked with anNH group, so that carriers can flow easily. Alternatively, the danglingbonds of the semiconductor atoms at the aforementioned interfaces areterminated with an NH₂ group, so that the defect level disappears. As aresult, resistance in a vertical direction (a thickness direction) isreduced when the thin film transistor is in an on state and voltage isapplied between the source and drain electrodes. That is, field-effectmobility and on current of the thin film transistor are increased.

By making the oxygen concentration of the second semiconductor layer 407lower than the nitrogen concentration of the second semiconductor layer407, bonds which interrupt carrier transfer in defects at the interfacebetween the microcrystalline semiconductor region 427A and the amorphoussemiconductor region 427B or at the interface between semiconductorcrystal grains can be reduced.

In this manner, off current of the thin film transistor can be reducedwhen a channel formation region is formed using the first semiconductorlayer 406 and the region 409 including an amorphous semiconductor isprovided between the channel formation region and the source and drainregions 410. In addition, off current can be further reduced while oncurrent and field-effect mobility can be increased when the mixed region408 and the region 409 including an amorphous semiconductor areprovided. This is because the mixed region 408 includes themicrocrystalline semiconductor region 427A having a conical or pyramidalshape and the region 409 including an amorphous semiconductor is formedusing a well-ordered semiconductor layer which has few defects and asteep tail of a level at a band edge in the valence band.

In order to control the threshold voltage (Vth), an impurity elementimparting p-type conductivity may be added to the first semiconductorlayer 406 serving as a channel formation region of the thin filmtransistor at the same time as or after formation of the firstsemiconductor layer 406. An example of an impurity element impartingp-type conductivity is boron, and a gas containing an impurity element,such as B₂H₆ or BF₃, may be mixed into silicon hydride at a proportionof 1 ppm to 1000 ppm inclusive, preferably 1 ppm to 100 ppm inclusive,whereby the first semiconductor layer 406 including an impurity elementimparting p-type conductivity can be formed. The concentration of boronincluded in the first semiconductor layer 406 may be preferably set tohigher than or equal to 1×10¹⁴ atoms/cm³ to lower than or equal to6×10¹⁶ atoms/cm³, for example.

The thickness of the first semiconductor layer 406 is preferably set togreater than or equal to 2 nm and less than or equal to 60 nm, morepreferably greater than or equal to 10 nm and less than or equal to 30nm. When the thickness of the first semiconductor layer 406 is in therange of from 2 nm to 60 nm, a thin film transistor can be made tooperate as a full depletion type thin film transistor. Note that thesecond semiconductor layer 407 may be formed to have a thickness ofgreater than or equal to 10 nm and less than or equal to 500 nm. Thethicknesses of these layers can be adjusted by a flow rate of silane andformation time, for example.

Note that it is preferable that an impurity element imparting oneconductivity type, such as phosphorus or boron, be not included in thesecond semiconductor layer 407. In the case where the secondsemiconductor layer 407 includes phosphorus, boron, or the like, theconcentration of phosphorus, boron, or the like is preferably adjustedto be lower than a lower detection limit of secondary ion massspectrometry.

This is for the prevention of formation of a PN junction at theinterface between the first semiconductor layer 406 and the secondsemiconductor layer 407 in the case where the first semiconductor layer406 includes boron and the second semiconductor layer 407 includesphosphorus. Further, this is for the prevention of formation of a PNjunction at the interface between the second semiconductor layer 407 andthe source and drain regions 410 in the case where the secondsemiconductor layer 407 includes boron and the source and drain regions410 include phosphorus. Furthermore, this is for the prevention ofgeneration of a recombination center and leakage current in the casewhere the second semiconductor layer 407 include both boron andphosphorus.

The second semiconductor layer 407 which does not include an impurityelement such as phosphorus or boron is provided between the source anddrain regions 410 and the first semiconductor layer 406, so that animpurity element can be prevented from entering the first semiconductorlayer 406 to be a channel formation region.

The source and drain regions 410 are provided in order that the secondsemiconductor layer 407 and the source and drain electrode layers 412have ohmic contact with each other. The source and drain regions 410 areformed using amorphous silicon to which phosphorus is added,microcrystalline silicon to which phosphorus is added, or the like. Notethat, in the case where a p-channel thin film transistor is formed as athin film transistor, the source and drain regions 410 are formed usingmicrocrystalline silicon to which boron is added, amorphous silicon towhich boron is added, or the like. There is no particular limitation oncrystallinity of the source and drain regions 410. The source and drainregions 410 may be formed using a crystalline semiconductor or anamorphous semiconductor; however, the source and drain regions 410 arepreferably formed using a crystalline semiconductor. This is because oncurrent is increased when the source and drain regions 410 are formedusing a crystalline semiconductor. Note that the source and drainregions 410 may be formed to have a thickness of greater than or equalto 2 nm and less than or equal to 60 nm.

A material of the source and drain electrode layers 412 are notparticularly limited as long as a conductive material is used. As theconductive material, a metal material such as molybdenum, titanium,chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandiumor an alloy material which includes any of these materials as a maincomponent can be used. Alternatively, crystalline silicon to which animpurity element imparting one conductivity type is added may be used.In addition, the source and drain electrode layers 412 may have a singlelayer structure or a stacked structure including a plurality of layers.For example, a two-layer structure in which a titanium layer or amolybdenum layer is stacked over an aluminum layer or a copper layer ora three-layer structure in which an aluminum layer or a copper layer issandwiched with titanium layers or molybdenum layers may be used.Alternatively, a titanium nitride layer may be used instead of thetitanium layer.

Next, a method for manufacturing a thin film transistor which is oneembodiment of the present invention is described.

The thin film transistor which is one embodiment of the presentinvention includes a crystalline semiconductor. An n-channel thin filmtransistor including a crystalline semiconductor has higher carriermobility than a p-channel thin film transistor including a crystallinesemiconductor. Further, it is preferable that all thin film transistorsformed over one substrate have the same polarity because the number ofmanufacturing steps can be reduced. Therefore, a method formanufacturing an n-channel thin film transistor is described here.However, this embodiment is not limited to this.

The gate electrode layer 402 is formed over the gate substrate 400.Then, the silicon nitride layer 404A which covers the gate electrodelayer 402 is formed (see FIG. 5A).

The gate electrode layer 402 can be formed in such a manner that aconductive layer is formed over the substrate 400 by a sputtering methodor a vacuum evaporation method; a resist mask is formed over theconductive layer by a photolithography method, an inkjet method, or thelike; and then the conductive layer is etched using the resist mask.Alternatively, the gate electrode layer 402 can be formed by discharginga conductive nanopaste of silver, gold, copper, or the like over thesubstrate by an inkjet method, and baking the conductive nanopaste. Notethat as a barrier metal for increasing adhesion between the gateelectrode layer 402 and the substrate 400 and preventing diffusion ofthe material used for the gate electrode layer 402 to a base, a nitridelayer of any of the above-described metal materials may be providedbetween the substrate 400 and the gate electrode layer 402. Here, aconductive layer is formed over the substrate 400 and etched using aresist mask formed using a photomask, so that the gate electrode layer402 is formed.

The gate electrode layer 402 is preferably processed to be tapered. Thisis because a semiconductor layer and a source wiring (a signal line) areto be formed over the gate electrode layer 402 in a later step. Inaddition, in this step, a gate wiring (a scan line) can be formed at thesame time. Note that a scan line refers to a wiring which selects apixel.

Note that, in a process for manufacturing a thin film transistordescribed below, a resist may be applied to an entire surface of asubstrate in a photolithography method. Alternatively, a resist isformed by a printing method over a region in which a resist mask is tobe formed and then the resist is exposed to light, whereby a resist canbe saved and cost can be reduced. Further alternatively, instead ofexposing a resist to light by using a light-exposure machine, a laserbeam direct drawing apparatus may be used to expose a resist to light.

The silicon nitride layer 404A is formed by a CVD method, a sputteringmethod, or the like. In the process for forming the silicon nitridelayer 404A by a CVD method, glow discharge plasma is generated bysupplying electrodes with high-frequency power with a frequency of 3 MHzto 30 MHz, typically 13.56 MHz or 27.12 MHz, or high-frequency power inthe VHF band with a frequency of 30 MHz to approximately 300 MHz,typically 60 MHz. Alternatively, high-frequency power with a microwavefrequency of 1 GHz or more may be used. With the use of high-frequencypower in the VHF band or at a microwave frequency, the deposition ratecan be increased. Note that the high-frequency power may be supplied ina pulsed manner or a continuous manner. In addition, by superimposinghigh-frequency power in the HF band and high-frequency power in the VHFband on each other, unevenness of plasma in a large-sized substrate isalso reduced, so that uniformity can be improved and the deposition ratecan be increased. When the silicon nitride layer 404A is formed using amicrowave plasma CVD apparatus with a frequency of greater than or equalto 1 GHz, withstand voltage between the gate electrode layer 402 and thesource and drain electrode layers 412 can be improved, whereby a highlyreliable thin film transistor can be obtained. Although the temperaturewhen the silicon nitride layer 404A is deposited can be set in the rangefrom room temperature to approximately 300° C., the temperature ispreferably set to greater than or equal to 260° C. and less than orequal to 300° C. to improve reliability of electric characteristics ofthe thin film transistor.

Then, the silicon oxide layer 404B is formed on the silicon nitridelayer 404A (see FIG. 5B). The gate insulating layer 404 can be formed bystacking the silicon nitride layer 404A and the silicon oxide layer404B. The silicon oxide layer 404B may have a thickness which enablesthe silicon oxide layer 404B to retain a film shape to cover the surfaceof the silicon nitride layer 404A and which does not reduce throughputextremely due to increase in time for etching the gate insulating layer404 which is performed to expose the gate electrode layer 402 to bedescribed later (that is, a thickness with which the time for etchingthe silicon oxide layer 404B is approximately the same as that foretching the silicon nitride layer 404A). The thickness of the siliconoxide layer 404B is preferably greater than or equal to 2 nm and lessthan 10 nm.

When the surface of the silicon nitride layer 404A is oxidized, a weakbond between silicon and nitrogen is cut, nitrogen is substituted byoxygen, and then oxygen is bonded to silicon. Alternatively, oxygen isbonded to dangling bonds in the silicon nitride layer. The silicon oxidelayer 404B can be formed on the surface of the silicon nitride layer404A through these reactions. In this embodiment, the surface of thesilicon oxide layer 404B is exposed to air, so that the surface of thesilicon nitride layer 404A can be oxidized. For example, the siliconnitride layer 404A which is formed over the substrate 400 may be exposedto air in a pretreatment chamber. Time for which the silicon nitridelayer 404A is exposed to air may be set to time for which the surface ofthe silicon nitride layer 404A can be oxidized. For example, the time ispreferably set to 1 minute to 24 hours. The longer the surface of thesilicon nitride layer 404A is exposed to air, the more crystallinity ofthe first semiconductor layer 406 is increased. Note that the siliconoxide layer 404B can be dense when the substrate 400 is heated while thesilicon nitride layer 404A is exposed to air to oxidize the surface ofthe silicon nitride layer 404A.

Next, the first semiconductor layer 406A is formed on the silicon oxidelayer 404B (see FIG. 5C). In this embodiment, the first semiconductorlayer 406A is formed using a microcrystalline semiconductor. In atreatment chamber of a plasma treatment apparatus, a deposition gascontaining silicon or germanium is mixed with hydrogen, glow dischargeplasma is generated, and thus the microcrystalline semiconductor layeris formed. Alternatively, the microcrystalline semiconductor layer isformed by generating glow discharge plasma with a mixture of adeposition gas containing silicon or germanium, hydrogen, and a rare gassuch as helium, neon, or krypton. Microcrystalline silicon,microcrystalline silicon germanium, microcrystalline germanium, or thelike is formed using a mixed gas which is obtained by diluting thedeposition gas containing silicon or germanium with hydrogen whose flowrate is 10 to 2000 times, preferably 10 to 200 times that of thedeposition gas. The deposition temperature at this time is higher thanor equal to room temperature and lower than or equal to 300° C.,preferably higher than or equal to 200° C. and lower than or equal to280° C.

Since the outermost surface of the gate insulating layer 404 is thesilicon oxide layer 404B, crystal growth of the first semiconductorlayer 406A is promoted from the surface of the gate insulating layer404. As a result, a microcrystalline semiconductor layer with highcrystallinity can be formed from the surface of the gate insulatinglayer 404 and thus on current and field-effect mobility of the thin filmtransistor can be increased. Further, when the silicon oxide layer 404Bis made thin, time for etching the gate insulating layer which isperformed to expose the gate electrode layer 402 is not made long,whereby throughput can be improved.

The thickness of the first semiconductor layer 406A is preferablygreater than or equal to 3 nm and less than or equal to 100 nm, morepreferably greater than or equal to 5 nm and less than or equal to 50nm. In the case where the first semiconductor layer 406A is too thin, oncurrent of the thin film transistor is reduced.

In the case where the first semiconductor layer 406A is too thick, offcurrent is increased when the thin film transistor operates at hightemperature. Therefore, the thickness of the first semiconductor layer406A is set to greater than or equal to 3 mm and less than or equal to100 nm, preferably greater than or equal to 5 nm to less than or equalto 50 nm, so that on current and off current of the thin film transistorcan be adjusted.

The glow discharge plasma used for forming the first semiconductor layer406A can be generated in the same manner as that used for forming thesilicon nitride layer 404A of the gate insulating layer 404.

As a typical example of the deposition gas containing silicon orgermanium, silane (SiH₄), disilane (Si₂H₆), germane (GeH₄), digermane(Ge₂H₆), or the like can be given.

A rare gas such as helium, argon, neon, krypton, or xenon is mixed intoa source gas of the first semiconductor layer 406A, whereby thedeposition rate of the first semiconductor layer 406A is increased. Whenthe deposition rate is increased, the amount of impurities mixed intothe first semiconductor layer 406A can be reduced, whereby thecrystallinity of the first semiconductor layer 406A can be improved.Accordingly, on current and field-effect mobility of the thin filmtransistor can be increased and throughput can also be increased.

Note that the first semiconductor layer 406A may be formed under two ormore different conditions. For example, after one part of the firstsemiconductor layer 406A is formed under a first condition, the otherpart of the first semiconductor layer 406A may be formed under acondition where a dilution ratio is lower than that used in the firstcondition. Alternatively, after one part of the first semiconductorlayer 406A is formed under a first condition, the other part of thefirst semiconductor layer 406A may be formed under a condition where adilution ratio is higher than that used in the first condition.

Note that before the first semiconductor layer 406A is formed,impurities in the treatment chamber of the CVD apparatus are removed byintroducing the deposition gas containing silicon or get manium whileexhausting a gas in the treatment chamber, so that the amount of theimpurities in the gate insulating layer 404 and the first semiconductorlayer 406A, which is formed later, of the thin film transistor can bereduced, and thus, electric characteristics of the thin film transistorcan be improved.

Next, a mixed region 408A and a region 409A including an amorphoussemiconductor are stacked over the first semiconductor layer 406A,whereby a second semiconductor layer 407A is formed (see FIG. 5D). Then,an impurity semiconductor layer 410A is formed over the secondsemiconductor layer 407A and a resist mask 420 is formed over theimpurity semiconductor layer 410A.

Since the first semiconductor layer 406A is formed using amicrocrystalline semiconductor layer, the second semiconductor layer407A is formed under a condition that crystals grow partly with themicrocrystalline semiconductor used as a seed crystal.

The second semiconductor layer 407A is formed by generating glowdischarge plasma with a mixture of a deposition gas containing siliconor germanium, hydrogen, and a gas containing nitrogen in a reactionchamber of a plasma CVD apparatus. Examples of the gas containingnitrogen include ammonia, nitrogen, nitrogen fluoride, nitrogenchloride, chloroamine, fluoroamine, and the like. Glow discharge plasmacan be generated in the same manner as that of the silicon nitride layer404A of the gate insulating layer 404.

In this case, a flow ratio of the deposition gas containing silicon orgermanium to hydrogen is the same as that for forming the firstsemiconductor layer 406A which is a microcrystalline semiconductor layerand a gas containing nitrogen is used as the source gas, whereby crystalgrowth can be suppressed as compared to the deposition condition of thefirst semiconductor layer 406A. Accordingly, the mixed region 408A andthe region 409A including an amorphous semiconductor can be formed. Notethat the region 409A including an amorphous semiconductor is awell-ordered semiconductor layer which has few defects and a steep tailof a level at a band edge in the valence band.

Here, an example of a condition for forming the second semiconductorlayer 407A is as follows: the flow rate of hydrogen is 10 to 2000 times,preferably 10 to 200 times that of the deposition gas containing siliconor germanium. Note that in an example of a normal condition for formingan amorphous semiconductor layer, the flow rate of hydrogen is 0 to 5times that of the deposition gas containing silicon or germanium.

A rare gas such as helium, neon, argon, xenon, or krypton is mixed intoa source gas of the second semiconductor layer 407A, whereby thedeposition rate of the second semiconductor layer 407A can be increased.

The thickness of the second semiconductor layer 407A is preferablygreater than or equal to 50 nm and less than or equal to 350 nm, morepreferably greater than or equal to 120 nm and less than or equal to 250nm.

At an early stage of deposition of the second semiconductor layer 407A,since a gas containing nitrogen is included in the source gas, thecrystal growth is partly suppressed; therefore, while a conical orpyramidal microcrystalline semiconductor region grows, an amorphoussemiconductor region is formed which fills a space between the conicalor pyramidal microcrystalline semiconductor regions. Such a region wherethe microcrystalline semiconductor region and the amorphoussemiconductor region are mixed is the mixed region 408A. Then, thecrystal growth of the conical or pyramidal microcrystallinesemiconductor region stops and only an amorphous semiconductor regionwhich does not include a microcrystalline semiconductor region isformed. Such a region where only the amorphous semiconductor regionwhich does not include a microcrystalline semiconductor region is formedis the region 409A including an amorphous semiconductor. Such conditionscorrespond to the “conditions that a crystal grows partly”. Before theconical or pyramidal microcrystalline semiconductor region grows, amicrocrystalline semiconductor layer may be deposited on the entiresurface of the first semiconductor layer 406 with the firstsemiconductor layer 406 used as a seed crystal.

Here, the second semiconductor layer 407A is formed using a mixture ofthe source gas of the second semiconductor layer 407A and a gascontaining nitrogen. Alternatively, the second semiconductor layer 407Aincluding the mixed region 408A and the region 409A including anamorphous semiconductor can be formed in a manner in which the surfaceof the first semiconductor layer 406A is exposed to a gas containingnitrogen, nitrogen is adsorbed onto the surface of the firstsemiconductor layer 406A, and glow discharge is performed using adeposition gas containing silicon or germanium and hydrogen as sourcegases.

Since the impurity semiconductor layer 410A serves as the source anddrain regions 410 illustrated in FIGS. 1A and 1B, the materialsdescribed as the materials of the source and drain regions 410 can beused as appropriate. The impurity semiconductor layer 410A can be formedusing a source gas introduced into the treatment chamber of the plasmaCVD apparatus, to which an impurity element imparting one conductivitytype is added. In the case where an n-channel thin film transistor isformed, for example, phosphorus may be added as the impurity element anda gas containing an impurity element imparting n-type conductivity, suchas phosphine (chemical formula: PH₃), may be added to silicon hydride.In the case where a p-channel thin film transistor is formed, forexample, boron may be added as the impurity element and a gas containingan impurity element imparting p-type conductivity, such as diborane(chemical formula: B₂H₆), may be added to silicon hydride.

Now, a formation process from the first semiconductor layer 406A up tothe impurity semiconductor layer 410A is described with reference to aschematic view of a plasma CVD apparatus (see FIG. 7) and a time chart.

A plasma CVD apparatus 161 illustrated in FIG. 7 is connected to a gassupply unit 150 and an exhaust unit 151 and includes a treatment chamber141, a stage 142, a gas supply portion 143, a shower plate 144, anexhaust port 145, an upper electrode 146, a lower electrode 147, analternate-current power source 148, and a temperature controller 149. Asubstrate 140 over which a film is to be formed is placed over the lowerelectrode 147.

The treatment chamber 141 is formed using a material having rigidity andthe inside thereof can be evacuated to vacuum. The treatment chamber 141is provided with the upper electrode 146 and the lower electrode 147.Note that in FIG. 7, a structure of a capacitive coupling type (aparallel plate type) is illustrated; however, another structure such asa structure of an inductive coupling type can be used, as long as plasmacan be generated in the treatment chamber 141 by supplying two or morekinds of high-frequency powers.

When treatment is performed with the plasma CVD apparatus illustrated inFIG. 7, a given gas is introduced from the gas supply portion 143. Theintroduced gas is introduced into the treatment chamber 141 through theshower plate 144. When high-frequency power is supplied by thealternate-current power source 148 connected to the upper electrode 146and the lower electrode 147 to excite the gas in the treatment chamber141, plasma is generated. Then, the gas in the treatment chamber 141 isexhausted through the exhaust port 145 connected to a vacuum pump.Further, the temperature controller 149 makes it possible to performplasma treatment while an object to be processed is being heated.

The gas supply unit 150 includes a cylinder 152 which is filled with agas, a pressure adjusting valve 153, a stop valve 154, a mass flowcontroller 155, and the like. The treatment chamber 141 includes theshower plate 144 between the upper electrode 146 and the substrate 140.The shower plate 144 is processed into a plate-like shape and providedwith a plurality of pores. The gas introduced into the upper electrode146 is introduced into the treatment chamber 141 from these pores of theshower plate 144 through an inner hollow structure.

The exhaust unit 151 connected to the treatment chamber 141 has afunction of vacuum evacuation and a function of controlling the pressureinside the treatment chamber 141 to be maintained at a predeterminedlevel when a reaction gas is made to flow. The exhaust unit 151 includesa valve 156, a conductance valve 157, a turbo molecular pump 158, a drypump 159, and the like. Although not illustrated, in the case ofarranging the valve 156 and the conductance valve 157 in parallel, thevalve 156 is closed and the conductance valve 157 is operated, so thatthe exhaust velocity is controlled and thus the pressure in thetreatment chamber 141 can be kept within a predetermined range. Notethat when the valve 156 having higher conductance is opened, thepressure in the treatment chamber 141 can be controlled to be maintainedat a predetermined level.

In the case where the treatment chamber 141 is evacuated to pressurelower than 10⁻⁵ Pa, a cryopump 160 is preferably used together.Alternatively, when exhaust is performed up to ultra-high vacuum asultimate degree of vacuum, the inner wall of the treatment chamber 141may be polished into a mirror surface and a heater for baking may beprovided in order to reduce gas emission from the inner wall.

Note that as illustrated in FIG. 7, when pre-coating treatment isperformed so that a film is formed (deposited) so as to cover the entireinner wall of the treatment chamber 141, it is possible to preventimpurities attached to the inner wall of the treatment chamber 141 orimpurities forming the inner wall of the treatment chamber 141 fromentering an element.

Note that for plasma to be generated, for example, RF (3 MHz to 30 MHz,for example, 13.56 MHz or 27 MHz) plasma, VHF (30 MHz to 300 MHz, forexample, 60 MHz) plasma, or microwave (1 GHz or higher, for example,2.45 GHz) plasma can be used. Note that plasma is preferably generatedin a pulsed manner.

In addition, a pretreatment chamber may be connected to the apparatus.When a substrate is preheated in the pretreatment chamber before filmformation, heating time required before the film formation in eachtreatment chamber can be shortened, whereby throughput can be increased.

Note that the use of a multi-chamber plasma CVD apparatus as the plasmaCVD apparatus allows a layer of one kind or layers with similar kinds ofcompositions to be formed in each chamber. Therefore, stacked films canbe formed without interfaces being contaminated by a residue of formedlayers or an impurity floating in the air.

Note that the inside of the treatment chamber 141 of the plasma CVDapparatus is preferably cleaned with fluorine radicals. Note also that aprotection film is preferably formed in the treatment chamber 141 beforefilm formation.

FIG. 8 is a time chart showing steps of forming the silicon nitridelayer 404A up to the impurity semiconductor layer 410A.

First, the substrate 400 over which the gate electrode layer 402 isformed is heated in the treatment chamber 141 of the plasma CVDapparatus and source gases used for forming the silicon nitride layer404A are introduced into the treatment chamber 141 (pretreatment 170 inFIG. 8). Here, as an example, a SiH₄ gas, an H₂ gas, an N₂ gas, and anNH₃ gas are introduced as source gases at flow rates of 40 sccm, 500sccm, 550 sccm, and 140 sccm, respectively, and are stabilized. Inaddition, the pressure in the treatment chamber 141 is set to 100 Pa:the substrate temperature is set to 280° C.; and plasma discharge isperformed using the RF power source frequency of 13.56 MHz and power ofthe RF power source of 370 W. Thus, the silicon nitride layer 404A witha thickness of approximately 300 nm is formed. After that, only theintroduction of the SiH₄ gas is stopped, and after several seconds (fiveseconds, here), the plasma discharge is stopped (silicon nitride layerformation 171 in FIG. 8). Note that either an N₂ gas or an NH₃ gas maybe used and in the case of mixing the gases to be used, the flow ratesof the gases may be adjusted as appropriate. Further, the flow rate ofthe H₂ gas is adjusted as appropriate when the H₂ gas is introduced, andthe H₂ gas is not necessarily introduced.

Next, the substrate is transferred from the treatment chamber 141 to aload lock chamber. The substrate is exposed to an air atmosphere, sothat the surface of the silicon nitride layer 404A is oxidized and thusthe silicon oxide layer 404B is formed on the silicon nitride layer 404A(substrate transfer 172 in FIG. 8).

Next, source gases used for forming the first semiconductor layer 406Aare introduced into the treatment chamber 141 (gas replacement 175 inFIG. 8).

Next, the first semiconductor layer 406A is formed over the entiresurface of the silicon oxide layer 404B. First, the source gases usedfor forming the first semiconductor layer 406A is introduced into thetreatment chamber 141. Here, as an example, a SiH₄ gas, an H₂ gas, andan Ar gas are introduced as source gases at flow rates of 10 sccm, 1500sccm, and 1500 sccm, respectively, and are stabilized. In addition, thepressure in the treatment chamber 141 is set to 280 Pa; the substratetemperature is set to 280° C.; and plasma discharge is performed usingthe RF power source frequency of 13.56 MHz and power of the RF powersource of 50 W. Thus, a microcrystalline silicon layer to be the firstsemiconductor layer 406A is formed. After that, in a manner similar tothat of the silicon nitride layer 404A or the like, only theintroduction of the SiH₄ gas is stopped, and after several seconds (fiveseconds, here), plasma discharge is stopped (silicon layer formation 176in FIG. 8). After that, these gases are exhausted, and gases used forforming the second semiconductor layer 407A are introduced (gasreplacement 177 in FIG. 8), Note that without being limited thereto,replacement of gases is not necessarily performed.

Next, the second semiconductor layer 407A is formed over the entiresurface of the first semiconductor layer 406A. First, the source gasesused for forming the second semiconductor layer 407A is introduced intothe treatment chamber 141. Here, as an example, a SiH₄ gas, an H₂ gas,and a NH₃, gas diluted with an H₂ gas to 1000 ppm are introduced assource gases at flow rates of 30 sccm, 1475 sccm, and 25 sccm,respectively, and are stabilized. In addition, the pressure in thetreatment chamber 141 is set to 280 Pa; the substrate temperature is setto 280° C.; and plasma discharge is performed using the RF power sourcefrequency of 13.56 MHz and power of the RF power source of 50 W. Thus, asilicon layer to be the second semiconductor layer 407A is formed. Afterthat, in a manner similar to that of the silicon nitride layer 404A orthe like, only the introduction of the SiH₄ gas is stopped, and afterseveral seconds (five seconds, here), the plasma discharge is stopped(second semiconductor layer formation 178 in FIG. 8). After that, thesegases are exhausted, and gases used for forming the impuritysemiconductor layer 410A containing an impurity element serving as adonor are introduced (gas replacement 179 in FIG. 8).

Next, the impurity semiconductor layer 410A containing an impurityelement serving as a donor is formed over the entire surface of thesecond semiconductor layer 407A. First, the source gases used forforming the impurity semiconductor layer 410A containing an impurityelement serving as a donor are introduced into the treatment chamber141. Here, as an example, a SiH₄ gas and a mixed gas in which a PH₃ gasis diluted with an H₂ gas to 0.5 vol % are introduced as the sourcegases at flow rates of 100 sccm and 170 sccm, respectively, and arestabilized. In addition, the pressure in the treatment chamber 141 isset to 170 Pa; the substrate temperature is set to 280° C.; and plasmadischarge is performed using the RF power source frequency of 13.56 MHz;and power of the RF power source of 60 W. Thus, the impuritysemiconductor layer 410A containing an impurity element serving as adonor is formed. After that, in a manner similar to that of the siliconnitride layer 404A or the like, only the introduction of the SiH₄ gas isstopped, and after several seconds (five seconds, here), plasmadischarge is stopped (impurity semiconductor layer formation 180 in FIG.8). After that, these gases are exhausted (exhaust 181 in FIG. 8). Whenthe flow ratio of the SiH₄ gas to the H₂ gas, which are used for formingthe impurity semiconductor layer 410A, is set to substantially the sameas that of the first semiconductor layer 406A, the impuritysemiconductor layer 410A can be formed of a crystalline semiconductor,which is preferable.

Thus, formation of the silicon nitride layer 404A up to the impuritysemiconductor layer 410A can be performed.

The resist mask 420 can be formed by a photolithography method.Alternatively, the resist mask 420 may be formed by an inkjet method orthe like.

Next, the first semiconductor layer 406A, the second semiconductor layer407A, and the impurity semiconductor layer 410A are etched using theresist mask 420. By this treatment, the first semiconductor layer 406A,the second semiconductor layer 407A, and the impurity semiconductorlayer 410A are separated into elements to form a stacked body 422 whichincludes the first semiconductor layer 406, a second semiconductor layer407B formed using the mixed region 408 and a region 409B including anamorphous semiconductor, and an impurity semiconductor layer 410B (seeFIG. 6A). Then, the resist mask 420 is removed.

This etching treatment is preferably performed so that the stacked body422 which includes the first semiconductor layer 406, the secondsemiconductor layer 407B, and the impurity semiconductor layer 410B canhave a tapered shape. The taper angle is from 30° to 90° inclusive,preferably from 40° to 80° inclusive. When the side surface has atapered shape, coverage with a layer which is formed thereover (e.g., awiring layer) in a later step can be improved. Accordingly,disconnection or the like caused by a step portion can be prevented.

Next, after a conductive layer 412A is formed over the impuritysemiconductor layer 410B and the gate insulating layer 404, a resistmask 424 is formed over the conductive layer 412A (see FIG. 6B). Theconductive layer 412A is to be patterned into the source and drainelectrode layers 412 in a later step.

The conductive layer 412A can be formed using a material similar to thatof the source and drain electrode layers 412 illustrated in FIGS. 1A and1B as appropriate. The conductive layer 412A is formed by a sputteringmethod, a vacuum evaporation method, or the like. Alternatively, theconductive layer 412A may be formed by discharging a conductivenanopaste of silver, gold, copper, or the like by a screen printingmethod, an inkjet method, or the like, and baking the conductivenanopaste.

In a manner similar to the resist mask 420, the resist mask 424 isformed by a photolithography method or an inkjet method.

Next, the conductive layer 412A is etched using the resist mask 424 toform the source and drain electrode layers 412. Wet etching ispreferably used. By this wet etching, portions of the conductive layer412A, which are not covered with the resist mask 424, is isotropicallyetched. The source and drain electrode layers 412 serve not only assource and drain electrodes of the thin film transistor but also as asource wiring (signal line).

Next, the impurity semiconductor layer 410B and the region 409Bincluding an amorphous semiconductor are etched with the resist mask 424formed thereover, so that the source and drain regions 410 and a backchannel portion (a portion illustrated as the back channel portion 432in FIG. 1B) are formed. Thus, the source and drain regions 410 areformed. Note that the region 409B including an amorphous semiconductoris etched with part thereof left, and thus the region 409 including anamorphous semiconductor is formed.

Here, etching may be performed by dry etching using a gas containingoxygen. The gas containing oxygen can etch the impurity semiconductorlayer 410B and the region 409B including an amorphous semiconductorwhile the resist mask is being recessed, so that the source and drainregions 410 and the region 409 including an amorphous semiconductor canhave a tapered shape. As the etching gas, for example, an etching gas inwhich oxygen is mixed into CF₄ or an etching gas in which oxygen ismixed into chlorine may be used. When the source and drain regions 410and the region 409 including an amorphous semiconductor have a taperedshape, electric field concentration can be prevented and off current canbe reduced.

Note that instead of the above steps, each of the conductive layer 412A,the impurity semiconductor layer 410B, and the region 409B including anamorphous semiconductor may be partly dry-etched. At this time, sidesurfaces of the source and drain electrode layers 412 are substantiallyaligned with side surfaces of the source and drain regions 410.

Alternatively, the conductive layer 412A may be etched using the resistmask 424 to form the source and drain electrode layers 412. Then, afterthe resist mask 424 is removed, the impurity semiconductor layer 410Band the region 409B including an amorphous semiconductor may be partlyetched. Since the impurity semiconductor layer 410B is etched using thesource and drain electrode layers 412 in this etching, side surfaces ofthe source and drain electrode layers 412 are substantially aligned withside surfaces of the source and drain regions 410.

The region 409B including an amorphous semiconductor is partly etched,so that the region 409 including an amorphous semiconductor has adepressed portion. It is preferable that the thickness of the region 409including an amorphous semiconductor be set so that the mixed region 408is not exposed and at least part of the region 409 including anamorphous semiconductor overlapping with the depressed portion is left.

After that, the resist mask 424 is removed (see FIG. 6C).

When residual products generated in the etching process, residues of theresist mask, a remover solution, and the like are attached to ordeposited over a surface of the region 409 including an amorphoussemiconductor which lies between the source and drain regions, offcurrent of the thin film transistor is increased. Therefore, for thepurpose of removing them, dry etching may be performed under a conditionwhich causes less damage, preferably without bias. Alternatively, plasmatreatment may be performed on a portion illustrated as the back channelportion 432 in FIG. 1B. Further alternatively, cleaning may beperformed. Furthermore, these steps may be combined.

Through the above steps, a channel-etched thin film transistor can bemanufactured.

After that, a connection terminal which is connected to the gateelectrode layer 402 can be formed by etching part of the gate insulatinglayer 404 to expose the gate electrode layer 402.

After the thin film transistor is manufactured through the above steps,an insulating layer is formed over the thin film transistor, part of theinsulating layer is etched to expose the source and drain electrodelayers 412 and the gate electrode layer 402, and a connection wiringwhich connects the source and drain electrode layers 412 to the gateelectrode layer 402 may be formed.

In these steps, the thickness of the silicon oxide layer 404B ispreferably greater than or equal to 2 nm and less than 10 nm, forexample, which does not reduce throughput extremely due to increase inthe time for etching the gate insulating layer 404 (that is, a thicknesswith which the time for etching the silicon oxide layer 404B isapproximately the same as that for etching the silicon nitride layer404A) which is performed to expose the gate electrode layer 402.

Through these steps, throughput in the steps for manufacturing the thinfilm transistor and the element substrate or the display deviceincluding the thin film transistor can be improved, and the thin filmtransistor with small off current, large on current, and highfield-effect mobility can be manufactured. Further, such a thin filmtransistor can be manufactured with high productivity.

Embodiment 2

The oxidation treatment for forming the silicon oxide layer 404B is notlimited to the oxidation treatment described in Embodiment 1. In thisembodiment, one embodiment of oxidation treatment is described in whichplasma is generated in an atmosphere containing oxygen and a surface tobe oxidized is exposed to the plasma.

First, similarly to Embodiment 1, the gate electrode layer 402 and thesilicon nitride layer 404A are formed over the substrate 400 (see FIG.5A). Then, plasma is generated in an atmosphere containing oxygen andthe silicon nitride layer 404A is exposed to the plasma. Accordingly,the surface of the silicon nitride layer 404A is oxidized to form thesilicon oxide layer 404B.

Here, the atmosphere which is used for generating plasma may contain atleast oxygen atoms. As an example of the atmosphere which is used forgenerating plasma, the following can be given: an oxygen atmosphere, anatmosphere containing nitrogen oxide (e.g., nitrogen monoxide, nitrogendioxide, dinitrogen monoxide, dinitrogen trioxide, dinitrogen tetroxide,or dinitrogen pentoxide), and an atmosphere containing sulfur oxide(e.g., sulfur monoxide, sulfur dioxide, or sulfur trioxide).Alternatively, a mixed gas atmosphere in which a rare gas (e.g., argon,neon, or xenon), nitrogen, or the like is added to any of the aboveatmospheres may be used. For example, an air atmosphere may be used. Itis preferable that a large amount of an oxygen gas or a gas containingoxygen be included in the atmosphere.

Alternatively, water plasma may be used. Water plasma can be generatedin such a manner that a gas containing water as its main component suchas water vapor is introduced into a treatment chamber. Note that waterplasma may be generated by introducing water vapor or the like into theabove atmosphere which is used for generating plasma.

As an apparatus which is used for generating plasma, for example, aplasma CVD apparatus can be given. Here, the plasma CVD apparatus whichis used for forming the silicon nitride layer 404A is preferably usedfor this plasma treatment; in this way, the same apparatus can be used.

Note that when an apparatus which can generate surface wave plasma isemployed as the apparatus for generating plasma, plasma treatment can beperformed without damages on the surface to be oxidized. Therefore, whenthe surface to be oxidized is the silicon nitride layer which is part ofthe gate insulating layer as in the case of Embodiment 1, damages on thesilicon nitride layer can be reduced by performing plasma treatment withthe use of surface wave plasma.

As described in this embodiment, a silicon oxide layer can be formed ona silicon nitride layer with the use of plasma including oxygen foroxidation treatment.

Embodiment 3

The oxidation treatment for forming the silicon oxide layer 404B is notlimited to the oxidation treatments described in Embodiments 1 and 2. Inthis embodiment, one embodiment of the oxidation treatment is described.

First, similarly to Embodiment 1, the gate electrode layer 402 and thesilicon nitride layer 404A are formed over the substrate 400 (see FIG.5A). Then, ozone is generated in a treatment chamber and the siliconnitride layer 404A is exposed to the ozone, so that the surface of thesilicon nitride layer 404A is oxidized to form the silicon oxide layer404B. Note that the substrate may be transferred into the treatmentchamber after ozone is generated in the treatment chamber.Alternatively, the substrate may be transferred into the treatmentchamber before ozone is generated.

Note that ozone means an isotope of oxygen, which consists of threeoxygen atoms.

Here, a method for generating ozone is not particularly limited andvarious methods can be used. For example, ozone can be generated byirradiation of an oxygen gas (or a gas containing oxygen) withultraviolet light or by electric discharge in an oxygen gas (or a gascontaining oxygen). In the case of using ultraviolet light, ultravioletlight having a short wavelength is preferably generated. For example,with the use of a mercury-vapor lamp, ultraviolet light having awavelength of 253.7 nm and a wavelength of 365.0 nm can be generated.Note that in the case of electric discharge, an oxygen gas (or a gascontaining oxygen) is introduced between two electrode plates (electrodeplates covered with a dielectric), high AC voltage is applied to causesilent electric discharge, oxygen molecules between the electrode platesare dissociated, and the oxygen atoms are recombined with other oxygenmolecule; thus, ozone can be generated.

Alternatively, ozone may be generated by electrolysis of dilute sulfuricacid or water, for example.

Further alternatively, air may be used instead of the above oxygen gas(or a gas containing oxygen). That is, ozone may be generated byirradiation of an air atmosphere with ultraviolet light or by electricdischarge in an air atmosphere.

As described in this embodiment, a silicon oxide layer can be formed ona silicon nitride layer with the use of ozone for oxidation treatment.

Embodiment 4

The oxidation treatment for forming the silicon oxide layer 404B is notlimited to the oxidation treatments described in Embodiments 1 to 3. Inthis embodiment, one embodiment of the oxidation treatment is described.

First, similarly to Embodiment 1, the gate electrode layer 402 and thesilicon nitride layer 404A are formed over the substrate 400 (see FIG.5A). Then, the substrate 400 is carried out of the treatment chamber,the silicon nitride layer 404A is exposed to ozone water to oxidize thesurface of the silicon nitride layer 404A; thus, the silicon oxide layer404B can be formed.

Note that ozone water is a solution of ozone, which is obtained bydissolving ozone in water. The ozone water can be formed by bubbling,for example. Note that ozone can be generated by the method described inEmbodiment 3.

Note that the concentration of the ozone water used here is notparticularly limited. The ozone water may have such a concentration thatthe silicon nitride layer 404A is oxidized. It is preferable that theozone water having a concentration of 1 ppm or more and 20 ppm or less,more preferably, 5 ppm or more and 15 ppm or less, be used.

As described in this embodiment, a silicon oxide layer can be formed ona silicon nitride layer with the use of ozone water for oxidationtreatment.

Embodiment 5

A silicon oxide layer may be formed over a silicon nitride layer usinganother method instead of the oxidation treatment described inEmbodiment 1. In this embodiment, one mode of the method is described.

First, similarly to Embodiment 1, the gate electrode layer 402 and thesilicon nitride layer 404A are formed over the substrate 400 (see FIG.5A). Then, the silicon oxide layer 404B is formed over the siliconnitride layer 404A. The silicon oxide layer 404B preferably has athickness which enables the silicon oxide layer 404B to retain a filmshape and which does not reduce throughput extremely in forming acontact hole. Therefore, the thickness of the silicon oxide layer ispreferably greater than or equal to 2 nm and less than or equal to 10nm.

Here, similarly to the silicon nitride layer 404A, the silicon oxidelayer 404B can be formed by a CVD method, preferably by a plasma CVDmethod, for example.

Note that the silicon oxide layer 404B can be formed usingtetraethoxysilane (TEOS (chemical formula: Si(OC₂H₅)₄)) as a source gas,for example. An atmospheric pressure CVD method or the like can be usedas a formation method.

Note that a silicon oxynitride layer may be used instead of the siliconoxide layer 404B formed over the silicon nitride layer 404A. Forexample, the silicon oxynitride layer can be formed as follows: SiH₄ andN₂O are introduced as source gases at flow rates of 5 sccm and 600 sccm,respectively, and are stabilized; the pressure in the treatment chamberis set at 25 Pa; the temperature is set at 280° C.; and plasma dischargeof 30 W is performed.

As described in this embodiment, a silicon oxide layer or a siliconoxynitride layer can be formed over a silicon nitride layer withoutusing oxidation treatment.

Embodiment 6

In this embodiment, a method for manufacturing a thin film transistor,which is one embodiment of the present invention and is different fromthose described in other embodiments, is described. Specifically, withthe use of a resist mask having regions with different thicknesses, thenumber of photomasks used in manufacturing a thin film transistor can bereduced.

First, similarly to Embodiment 2, a gate insulating layer 704 is formedso as to cover a gate electrode layer 702, and a first semiconductorlayer 706A, a second semiconductor layer 707A, and an impuritysemiconductor layer 710A are formed over the gate insulating layer 704.Then, a conductive layer 712A is formed over the impurity semiconductorlayer 710A. Note that the gate insulating layer 704 is formed includinga silicon nitride layer 704A and a silicon oxide layer 704B. Note thatthe second semiconductor layer 707A is formed including a mixed region708A and a region 709A including an amorphous semiconductor.

Next, a resist mask 720 is formed over the conductive layer 712A (seeFIG. 10A). The resist mask 720 in this embodiment is a resist maskhaving a depressed portion and a projected portion. In other words, theresist mask 720 can also be referred to as a resist mask including aplurality of regions (here, two regions) with different thicknesses. Aregion of the resist mask 720, which has a larger thickness, is referredto as a projected portion of the resist mask 720. A region of the resistmask 720, which has a smaller thickness, is referred to as a depressedportion of the resist mask 720.

The projected portion of the resist mask 720 is formed in a region wheresource and drain electrode layers 712 are to be formed. The depressedportion of the resist mask 720 is formed in a region where the sourceand drain electrode layers 712 are not to be formed and the secondsemiconductor layer 707A is to be exposed.

The resist mask 720 can be formed using a multi-tone mask. Here,multi-tone masks are described below with reference to FIGS. 12A and12B.

The multi-tone mask is a mask capable of light exposure with multi-levellight intensity. For example, light exposure is performed with threelevels of light intensity to provide an exposed region, a half-exposedregion, and an unexposed region. With the use of the multi-tone mask,one-time light exposure and development process allow a resist mask withplural thicknesses (for example, two levels of thicknesses) to beformed. Therefore, with the use of a multi-tone mask, the number ofphotomasks can be reduced.

FIGS. 12A and 12B illustrate cross-sectional views of typical multi-tonemasks. FIG. 12A illustrates a gray-tone mask 740 and FIG. 12Billustrates a half-tone mask 745.

The gray-tone mask 740 illustrated in FIG. 12A includes a light-blockingportion 742 formed using a light-blocking film on a substrate 741 havinga light-transmitting property, and a diffraction grating portion 743provided with a pattern of the light-blocking film.

The diffraction grating portion 743 has slits, dots, meshes, or the likewhich are provided at intervals less than or equal to the resolutionlimit of light used for light exposure, whereby the amount of lighttransmitted through the diffraction grating portion 743 is adjusted.Note that the slits, dots, or meshes provided at the diffraction gratingportion 743 may be provided periodically or non-periodically.

As the substrate 741 having a light-transmitting property, quartz or thelike can be used. The light-blocking film for forming the light-blockingportion 742 and the diffraction grating portion 743 may be formed usinga metal material, and chromium, chromium oxide, or the like ispreferably used.

In the case where the gray-tone mask 740 is irradiated with light forlight exposure, as illustrated in FIG. 12A, the light transmittance ofthe region overlapping with the light-blocking portion 742 is 0%, andthe light transmittance of the region where neither the light-blockingportion 742 nor the diffraction grating portion 743 is provided is 100%.Further, the light transmittance at the diffraction grating portion 743is approximately in the range of 10% to 70%, which can be adjusted bythe interval of slits, dots, meshes, or the like of the diffractiongrating.

The half-tone mask 745 illustrated in FIG. 12B includes asemi-light-transmitting portion 747 formed using asemi-light-transmitting film on a substrate 746 having alight-transmitting property, and a light-blocking portion 748 formedusing a light-blocking film.

The semi-light-transmitting portion 747 can be formed using a film ofMoSiN, MoSi, MoSiO, MoSiON, CrSi, or the like. The light-blockingportion 748 may be formed using a metal material similar to the materialof the light-blocking film of the gray-tone mask, and chromium, chromiumoxide, or the like is preferably used.

In the case where the half-tone mask 745 is irradiated with light forlight exposure, as illustrated in FIG. 12B, the light transmittance ofthe region overlapping with the light-blocking portion 748 is 0%, andthe light transmittance of the region where neither the light-blockingportion 748 nor the semi-light-transmitting portion 747 is provided is100%. Further, the light transmittance in the semi-light-transmittingportion 747 is approximately in the range of 10% to 70%, which can beadjusted by the kind, the thickness, or the like of the material to beformed.

By light exposure and development with the use of the multi-tone mask,the resist mask 720 which has regions with different thicknesses can beformed. Note that without limitation thereto, the resist mask 720 may beformed without using a multi-tone mask.

Next, etching is performed using the resist mask 720 to form a firstsemiconductor layer 706, a second semiconductor layer 707B, an impuritysemiconductor layer 710B, and a conductive layer 712B (see FIG. 10B).Note that the second semiconductor layer 707B is formed including amixed region 708B and a region 709B including an amorphoussemiconductor.

Next, the resist mask 720 is made to recede (reduce), so that resistmasks 724 are formed (see FIG. 10B). In order to make the resist mask720 recede (reduce), ashing using oxygen plasma or the like may beperformed.

Next, the conductive layer 712B is etched using the resist masks 724 toform the source and drain electrode layers 712 (see FIG. 10C).

Next, the impurity semiconductor layer 710B and the second semiconductorlayer 707B are partly etched, so that source and drain regions 710 and aregion 709 including an amorphous semiconductor are formed (see FIG.11A). After that, the resist masks 724 are removed (see FIG. 11B).

As described in this embodiment, a thin film transistor can bemanufactured using a multi-tone mask. Accordingly, the use of themulti-tone mask makes it possible to reduce the number of photomasks tobe used.

Embodiment 7

In this embodiment, one mode of a display panel or a light-emittingpanel is described with reference to drawings.

In the display device or the light-emitting device of this embodiment, asignal line driver circuit and a scan line driver circuit may be formedover a different substrate (e.g., a semiconductor substrate or an SOIsubstrate) and then connected to a pixel portion, or may be formed overthe same substrate as a pixel circuit.

Note that there are no particular limitations on the connection methodof a substrate separately formed, and a known method such as a COGmethod, a wire bonding method, or a TAB method can be used. Further, aconnection position is not limited to a particular position as long aselectrical connection is possible. Moreover, a controller, a CPU, amemory, and/or the like may be formed separately and connected to thepixel circuit.

FIG. 13 illustrates a block diagram of a display device. The displaydevice illustrated in FIG. 13 includes a pixel portion 800 including aplurality of pixels each provided with a display element, a scan linedriver circuit 802 which selects each pixel, and a signal line drivercircuit 803 which controls input of a video signal to a selected pixel.

Note that the display device is not limited to the mode illustrated inFIG. 13. That is, the signal line driver circuit is not limited to amode including only a shift register and an analog switch. In additionto the shift register and the analog switch, another circuit such as abuffer, a level shifter, or a source follower may be included. Further,the shift register and the analog switch are not always required to beprovided, and for example, a different circuit such as a decoder circuitby which a signal line can be selected may be used instead of the shiftregister, or a latch or the like may be used instead of the analogswitch.

The signal line driver circuit 803 illustrated in FIG. 13 includes ashift register 804 and an analog switch 805. A clock signal (CLK) and astart pulse signal (SP) are input to the shift register 804. When theclock signal (CLK) and the start pulse signal (SP) are input, timingsignals are generated in the shift register 804, and the timing signalsare input to the analog switch 805.

A video signal is supplied to the analog switch 805. The analog switch805 samples the video signal in accordance with the input timing signaland distributes the video signal to a signal line of a latter stage.

The scan line driver circuit 802 illustrated in FIG. 13 includes a shiftregister 806 and a buffer 807. The scan line driver circuit 802 mayinclude a level shifter. In the scan line driver circuit 802, when theclock signal (CLK) and the start pulse signal (SP) are input to theshift register 806, a selection signal is generated. The generatedselection signal is buffered and amplified in the buffer 807, and thensupplied to a corresponding scan line. Gates of all pixel transistors ofone line are connected to one scan line. Further, since the pixeltransistors of one line should be turned on at the same time in theoperation, a buffer through which large current can flow is used as thebuffer 807.

In a full-color display device, when video signals corresponding to R(red), G (green), and B (blue) are sequentially sampled and supplied toa corresponding signal line, the number of terminals for connecting theshift register 804 and the analog switch 805 corresponds toapproximately ⅓ of the number of terminals for connecting the analogswitch 805 and the signal lines of the pixel portion 800. Accordingly,by forming the analog switch 805 and the pixel portion 800 over the samesubstrate, the number of terminals which are used for connection to asubstrate that is formed separately can be reduced as compared to thecase of forming the analog switch 805 and the pixel portion 800 overdifferent substrates. Thus, occurrence probability of poor connectioncan be suppressed, and a yield can be improved.

Note that although the scan line driver circuit 802 in FIG. 13 includesthe shift register 806 and the buffer 807, without limitation thereto,the scan line driver circuit 802 may be formed using only the shiftregister 806.

Note that the structures of the signal line driver circuit and the scanline driver circuit are not limited to the structures illustrated inFIG. 13, which are merely one mode of the display device.

Next, appearances of a liquid crystal display panel and a light-emittingpanel, each of which is one mode of the display device, are describedwith reference to FIGS. 14A and 14B and FIGS. 15A and 15B. FIG. 14A is atop view of a liquid crystal display panel in which a transistor 820including a crystalline semiconductor and a liquid crystal element 823which are formed over a first substrate 811 are sealed with a sealant815 between the first substrate 811 and a second substrate 816. FIG. 14Bis a cross-sectional view taken along line K-L in FIG. 14A. FIGS. 15Aand 15B illustrate a case of a light-emitting panel. In FIGS. 15A and15B, only portions which are different from those in FIGS. 14A and 14Bare denoted by reference numerals.

The sealant 815 is provided so as to surround a pixel portion 812 and ascan line driver circuit 814 which are provided over the first substrate811. Further, the second substrate 816 is provided over the pixelportion 812 and the scan line driver circuit 814. Thus, the pixelportion 812 and the scan line driver circuit 814, together with a liquidcrystal layer 818 or a filler 831, are sealed with the first substrate811, the sealant 815, and the second substrate 816. Note that a signalline driver circuit 813 is mounted on a region over the first substrate811, which is different from the region surrounded by the sealant 815.Note that the signal line driver circuit 813 is formed using atransistor including a crystalline semiconductor formed over a substrateseparately prepared. Note that although an example in which the signalline driver circuit 813 formed using a transistor including acrystalline semiconductor is attached to the first substrate 811 isdescribed in this embodiment, a signal line driver circuit is preferablyformed using a transistor including a single crystal semiconductor andattached. FIG. 14B illustrates, as an example, a transistor 819 formedusing a crystalline semiconductor, which is included in the signal linedriver circuit 813.

The pixel portion 812 provided over the first substrate 811 includes aplurality of thin film transistors, and in FIG. 14B, a transistor 820included in the pixel portion 812 is illustrated. In addition, thesignal line driver circuit 813 also includes a plurality of thin filmtransistors, and in FIG. 14B, the transistor 819 included in the signalline driver circuit 813 is illustrated. In the light-emitting device ofthis embodiment, the transistor 820 may be a driving transistor, acurrent control transistor, or an erasing transistor. The transistor 820corresponds to the transistor described in Embodiment 1.

Note that a pixel electrode 822 included in the liquid crystal element823 is electrically connected to the transistor 820 through a wiring828. A counter electrode 827 of the liquid crystal element 823 is formedon the second substrate 816. The liquid crystal element 823 correspondsto a region where the pixel electrode 822, the counter electrode 827,and the liquid crystal layer 818 are stacked.

Further, a pixel electrode included in a light-emitting element 830 iselectrically connected to a source or drain electrode of the transistor820 through a wiring. In addition, in this embodiment, a commonelectrode of the light-emitting element 830 and a conductive materiallayer having a light-transmitting property are electrically connected toeach other. Note that the structure of the light-emitting element 830 isnot limited to that described in this embodiment. The structure of thelight-emitting element 830 can be determined in accordance with thedirection of light taken from the light-emitting element 830, polarityof the transistor 820, or the like.

Note that as a material of each of the first substrate 811 and thesecond substrate 816, glass, metal (typically stainless steel),ceramics, plastics, or the like can be used. As plastics, a fiberglassreinforced plastic (FRP) plate, a polyvinyl fluoride (PVF) film, apolyester film, an acrylic resin film, or the like can be used.Alternatively, a sheet in which aluminum foil is interposed between PVFfilms or polyester films may also be used.

A spacer 821 is a bead spacer and is provided to control a distance (acell gap) between the pixel electrode 822 and the counter electrode 827.Further, a spacer (a post spacer) which is obtained by selectivelyetching an insulating layer may also be used.

A variety of signals (potentials) are supplied to the signal line drivercircuit 813 which is formed separately, the scan line driver circuit814, and the pixel portion 812 from a flexible printed circuit (FPC) 817through a lead wiring 824 and a lead wiring 825.

In this embodiment, a connection terminal 826 is formed of the sameconductive layer as the pixel electrode 822 included in the liquidcrystal element 823. Further, the lead wiring 824 and the lead wiring825 are formed of the same conductive layer as the wiring 828.

The connection terminal 826 is electrically connected to a terminalincluded in the FPC 817 through an anisotropic conductive layer 829.

Note that although not illustrated, the liquid crystal display devicedescribed in this embodiment includes alignment films and polarizingplates, and may also include a color filter, a light-blocking layer, orthe like.

In this embodiment, the connection terminal 826 is formed of the sameconductive layer as the pixel electrode included in the light-emittingelement 830. In addition, the lead wiring 825 is formed of the sameconductive layer as the wiring 828. However, the present invention isnot limited to this embodiment.

As the second substrate located in the direction in which light isextracted from the light-emitting element 830, a light-transmittingsubstrate is used. In this case, a substrate formed of alight-transmitting material such as a glass substrate, a plasticsubstrate, a polyester film, or an acrylic film is used. When light isextracted from the light-emitting element 830 in the direction of thefirst substrate, a light-transmitting substrate is used as the firstsubstrate.

As the filler 831, an inert gas such as nitrogen or argon, anultraviolet curable resin, a thermosetting resin, or the like can beused. For example, polyvinyl chloride (PVC), acrylic, polyimide, anepoxy resin, a silicone resin, polyvinyl butyral (PVB), or ethylenevinyl acetate (EVA) can be used. In this embodiment, for example,nitrogen may be used.

An optical film such as a polarizing plate, a circular polarizing plate(including an elliptical polarizing plate), a retardation plate (aquarter-wave plate or a half-wave plate), or a color filter may beprovided as appropriate over a light-emitting surface of thelight-emitting element. Further, a polarizing plate or a circularlypolarizing plate may be provided with an anti-reflection layer.

Embodiment 8

The present invention disclosed in Embodiments 1 to 7 can be applied toa variety of electronic devices (including game machines). Examples ofelectronic devices include a television set (also referred to as atelevision or a television receiver), a monitor of a computer,electronic paper, a camera such as a digital camera or a digital videocamera, a digital photo frame, a mobile phone set (also referred to as acellular phone or a mobile phone device), a portable game console, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.

The present invention disclosed in Embodiments 1 to 7 can be applied to,for example, electronic paper. Electronic paper can be used forelectronic devices of a variety of fields as long as they display data.For example, electronic paper can be applied to an electronic book(e-book) reader, a poster, an advertisement in a vehicle such as atrain, displays of various cards such as a credit card, and the like. Anexample of such electronic devices is illustrated in FIG. 16A.

FIG. 16A illustrates an example of an electronic book reader. Theelectronic book reader illustrated in FIG. 16A includes a housing 900and a housing 901. The housing 900 and the housing 901 are combined witheach other by a hinge 904 so that the electronic book can be opened andclosed. With such a structure, the electronic book reader can be handledlike a paper book.

A display portion 902 is incorporated in the housing 900, and a displayportion 903 is incorporated in the housing 901. The display portion 902and the display portion 903 may display one image or different images.In the structure where different images are displayed on the displayportion 902 and the display portion 903, for example, the right displayportion (the display portion 902 in FIG. 16A) can display text and theleft display portion (the display portion 903 in FIG. 16A) can displayimages. The display device described in Embodiment 7 can be applied tothe display portions 902 and 903.

FIG. 16A illustrates an example in which the housing 900 is providedwith an operation portion and the like. For example, the housing 900 isprovided with a power input terminal 905, an operation key 906, aspeaker 907, and the like. The page can be turned with the operation key906, for example. Note that a keyboard, a pointing device, or the likemay be provided on the surface of the housing, on which the displayportion is provided. Further, an external connection terminal (e.g., anearphone terminal, a USB terminal, or a terminal which can be connectedto a variety of cables such as a USB cable), a recording mediuminsertion portion, or the like may be provided on a back surface or aside surface of the housing. Furthermore, the electronic book readerillustrated in FIG. 16A may have a function of an electronic dictionary.

The electronic book reader illustrated in FIG. 16A may be configured totransmit and receive data wirelessly. Through wireless communication,desired book data or the like can be purchased and downloaded from anelectronic book server.

FIG. 16B illustrates an example of a digital photo frame. For example,in the digital photo frame illustrated in FIG. 16B, a display portion912 is incorporated in a housing 911. The display portion 912 candisplay a variety of images. For example, the display portion 912 candisplay data of an image shot with a digital camera or the like andfunction as a normal photo frame. The display device described inEmbodiment 7 can be applied to the display portion 912.

Note that the digital photo frame illustrated in FIG. 16B may beprovided with an operation portion, an external connection terminal (aUSB terminal, a terminal which can be connected to a variety of cablessuch as a USB cable, and the like), a recording medium insertionportion, and the like. Although these components may be provided on thesurface on which the display portion is provided, it is preferable toprovide them on the side surface or the back surface for the design ofthe digital photo frame. For example, a memory storing data of an imageshot with a digital camera is inserted in the recording medium insertionportion of the digital photo frame, whereby the image data can bedownloaded and displayed on the display portion 912.

The digital photo frame illustrated in FIG. 16B may be configured totransmit and receive data wirelessly. The structure may be employed inwhich desired image data is transferred wirelessly to be displayed.

FIG. 16C illustrates an example of a television set. In the televisionset illustrated in FIG. 16C, a display portion 922 is incorporated in ahousing 921. Images can be displayed on the display portion 922. Here,the housing 921 is supported by a stand 923. The display devicedescribed in Embodiment 7 can be applied to the display portion 922.

The television set illustrated in FIG. 16C can be operated with anoperation switch of the housing 921 or a separate remote controller.Channels and volume can be controlled by an operation key of the remotecontroller so that an image displayed on the display portion 922 can becontrolled. Further, the remote controller may be provided with adisplay portion for displaying data output from the remote controller.

Note that the television set illustrated in FIG. 16C is provided with areceiver, a modem, and the like. With the receiver, a general televisionbroadcast can be received. Further, when the television set is connectedto a communication network by wired or wireless connection via themodem, one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver or between receivers) data communication canbe performed.

FIG. 16D illustrates an example of a mobile phone set. The mobile phoneset illustrated in FIG. 16D is provided with a display portion 932incorporated in a housing 931, operation buttons 933 and 937, anexternal connection port 934, a speaker 935, a microphone 936, and thelike. The display device described in Embodiment 7 can be applied to thedisplay portion 932.

The display portion 932 of the mobile phone set illustrated in FIG. 16Dmay be a touch panel. When the display portion 932 is touched with afinger or the like, contents displayed on the display portion 932 can becontrolled. In this case, operations such as making a phone call andtexting can be performed by touching the display portion 932 with afinger or the like.

There are mainly three screen modes of the display portion 932. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a phone call or texting, a text inputmode mainly for inputting text is selected for the display portion 932and text input operation can be performed on the screen. In this case,it is preferable to display a keyboard or number buttons on almost theentire screen of the display portion 932.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone set illustrated in FIG. 16D, display on the screen of thedisplay portion 932 can be automatically switched by determining theorientation of the mobile phone set (whether the mobile phone set isplaced horizontally or vertically for a landscape mode or a portraitmode).

The screen modes may be switched by touching the display portion 932 oroperating the operation button 937 of the housing 931. Alternatively,the screen modes may be switched depending on the kind of the imagedisplayed on the display portion 932. For example, when a signal of animage displayed on the display portion is the one of moving image data,the screen mode is switched to the display mode. When the signal is theone of text data, the screen mode is switched to the input mode.

Further, in the input mode, when input by touching the display portion932 is not performed for a certain period while a signal is detected byan optical sensor in the display portion 932, the screen mode may becontrolled so as to be switched from the input mode to the display mode.

The display portion 932 can also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenby an image sensor when the display portion 932 is touched with a palmor a finger, whereby personal authentication can be performed. Further,when a backlight that emits near-infrared light or a sensing lightsource that emits near-infrared light is provided in the displayportion, an image of a finger vein, a palm vein, or the like can betaken.

FIGS. 17A to 17C illustrate an example of a mobile phone. FIG. 17A is afront view, FIG. 17B is a rear view, and FIG. 17C is a front view inwhich two housings are slid. The mobile phone illustrated in FIGS. 17Ato 17C includes two housings: a housing 951 and a housing 952. Themobile phone has both functions of a mobile phone and a portableinformation terminal and incorporates a computer, which is a so-calledsmartphone capable of a variety of data processing in addition to voicecalls.

The housing 951 includes a display portion 953, a speaker 954, amicrophone 955, operation keys 956, a pointing device 957, a frontcamera lens 958, a jack 959 for an external connection terminal, anearphone terminal 960, and the like. The housing 952 includes a keyboard961, an external memory slot 962, a rear camera 963, a light 964, andthe like. In addition, an antenna is built in the housing 951.

Further, in addition to the above components, a non-contact IC chip, asmall size memory device, or the like may be incorporated in the mobilephone.

The housings 951 and 952 which overlap with each other (illustrated inFIG. 17A) can be slid and are developed by being slid as illustrated inFIG. 17C. The display panel or the display device to which the inventiondescribed in any of Embodiments 1 to 7 is applied can be incorporated inthe display portion 953. Since the front camera lens 958 is provided inthe same plane as the display portion 953, the mobile phone can be usedas a videophone. Further, a still image and a moving image can be takenwith the rear camera 963 and the light 964 by using the display portion953 as a viewfinder.

By using the speaker 954 and the microphone 955, the mobile phone can beused as an audio recording device (sound recorder) or an audioreproducing device. With the use of the operation keys 956, operation ofincoming and outgoing of phone calls, simple information input such aselectronic mail, scrolling of a screen, cursor movement for selectinginformation to be displayed on the display portion, and the like arepossible.

When much information is handled in documentation, use as a portableinformation terminal, and the like, it is convenient to use the keyboard961. By sliding the housings 951 and 952 which overlap with each other(FIG. 17A), the housings 951 and 952 can be developed as illustrated inFIG. 17C. When the mobile phone is used as a portable informationterminal, a cursor can be moved smoothly with the use of the keyboard961 and the pointing device 957. An AC adaptor and various types ofcables such as a USB cable can be connected to the jack 959 for anexternal connection terminal, and charging and data communication with apersonal computer or the like are possible. Further, by inserting arecording medium in the external memory slot 962, a larger amount ofdata can be stored and transferred.

The rear surface of the housing 952 (FIG. 17B) is provided with the rearcamera 963 and the light 964, and a still image and a moving image canbe taken using the display portion 953 as a viewfinder.

Further, the mobile phone may have an infrared communication function, aUSB port, a function of receiving one segment television broadcast, anon-contact IC chip, an earphone jack, or the like, in addition to theaforementioned functions and structures.

As described above, the invention disclosed in Embodiments 1 to 7 can beapplied to a variety of electronic devices.

Example 1

In Embodiment 1, exposure to air was described as an example ofoxidation treatment on a silicon nitride layer. In this example, aresult is described which is obtained by evaluating a silicon nitridelayer which is exposed to air after formed over a substrate by X-rayphotoelectron spectroscopy (XPS). Note that depth measured by the XPS inthis example was approximately 2 nm.

In this example, the thickness of the silicon nitride layer was 100 nm.The silicon nitride layer was formed as follows: a SiH₄ gas, an NH₃ gas,and an Ar gas were introduced as source gases at flow rates of 5 sccm,400 sccm, and 50 sccm, respectively, and were stabilized; the pressurein the treatment chamber was set to 30 Pa; the substrate temperature wasset to 280° C.; and plasma discharge of 500 W was performed using the RFpower source frequency of 27 MHz. Note that a parallel plate plasma CVDapparatus was used in film formation, in which a distance between anupper electrode and a lower electrode was 20 mm.

FIG. 18 shows a photoelectron spectroscopy spectrum of Si-2p, In FIG.18, the horizontal axis represents the bond energy of Si-2p and thevertical axis represents the spectrum intensity. Since the peak positionof the photoelectron spectroscopy spectrum is determined depending onthe electron state of elements, the peak position depends on the bondingstate. The bonding state of Si can be confirmed from the photoelectronspectroscopy spectrum of Si-2p in FIG. 18.

A peak A in FIG. 18 is derived from Si—O bonding (derived from SiO₂) anda peak B is derived from Si—N bonding (derived from Si₃N₄). Since thephotoelectron spectroscopy spectrum of Si-2p in FIG. 18 includes thepeak derived from the Si—O bonding, it can be said that a surface of thesilicon nitride layer includes the Si—O bonding. Accordingly, it can besaid that the surface of the silicon nitride layer is oxidized.

Example 2

In this example, nitrogen concentrations and oxygen concentrations ofthe interface between the gate insulating layer 404 and the firstsemiconductor layer 406, the first semiconductor layer 406, the secondsemiconductor layer 407, and the impurity semiconductor layer serving asthe source and drain regions 410, all of which are included in the thinfilm transistor illustrated in FIG. 1A, are described with reference toFIG. 19.

First, a process for forming a sample is described. Note that formationconditions of only the gate insulating layer 404, the firstsemiconductor layer 406, the second semiconductor layer 407, and theimpurity semiconductor layer serving as the source and drain regions 410are described here.

A silicon nitride layer with a thickness of 300 nm was formed over aglass substrate.

Here, as the glass substrate, a glass substrate with a thickness of 0.7mm (EAGLE2000, manufactured by Corning, Inc.) was used.

The silicon nitride layer was formed under following conditions: as forthe source gases, the flow rate of SiH₄ was set to 40 sccm, the flowrate of H₂ was set to 500 sccm, the flow rate of N₂ was set to 550 sccm,and the flow rate of NH₃ was set to 140 sccm; the pressure in thetreatment chamber was set to 100 Pa; the substrate temperature was setto 280° C.; and plasma discharge was performed using the RF power sourcefrequency of 13.56 MHz and power of the RF power source of 370 W.

Next, the glass substrate was transferred to a load lock chamber, thesilicon nitride layer was exposed to air for approximately 5 minutes,and thus the surface of the silicon nitride layer was oxidized to form asilicon oxide layer. Then, after cleaning of the treatment chamber ofthe plasma CVD apparatus, a protective layer was formed.

Here, after the treatment chamber was cleaned with nitrogen fluoride, anamorphous silicon layer was formed as a protective layer on an innerwall of the treatment chamber. The amorphous silicon layer was formed asfollows: the flow rate of SiH₄, which was a source gas, was set to 300sccm; the pressure in the treatment chamber was set to 160 Pa; thetemperature in the treatment chamber was set to 280° C.; and plasmadischarge was performed using the RF power source frequency of 13.56 MHzand power of the RF power source of 120 W.

Next, after the glass substrate was transferred to the treatmentchamber, the first semiconductor layer 406, the second semiconductorlayer 407, and the impurity semiconductor layer were stacked over thesilicon oxide layer in this order. Here, a microcrystalline siliconlayer with a thickness of 30 nm was formed as the first semiconductorlayer 406. A silicon layer with a thickness of 175 nm was formed as thesecond semiconductor layer 407. An amorphous silicon layer includingphosphorus with a thickness of 50 nm was formed as the impuritysemiconductor layer.

The first semiconductor layer 406 was formed under following conditions:as for the source gases, the flow rate of a SiH₄ gas was set to 5 sccm,the flow rate of an H₂ gas was set to 1500 sccm, and the flow rate of anAr gas was set to 1500 sccm; the pressure in the treatment chamber wasset to 280 Pa; the substrate temperature was set to 280° C.; and plasmadischarge was performed using the RF power source frequency of 13.56 MHzand power of the RF power source of 50 W.

The second semiconductor layer 407 was formed under followingconditions: as for the source gases, the flow rate of a SiH₄ gas was setto 40 sccm, the flow rate of an H₂ gas was set to 1475 sccm, the flowrate of 1000 ppm NH₃ gas (diluted with an H₂ gas) was set to 25 sccm,and the flow rate of an Ar gas was set to 2000 sccm; the pressure in thetreatment chamber was set to 280 Pa; the substrate temperature was 280°C.; and plasma discharge was performed using the RF power sourcefrequency of 13.56 MHz and power of the RF power source of 100 W.

The impurity semiconductor layer was formed under following conditions:as for source gases, the flow rate of SiH₄ was set to 100 sccm and theflow rate of 0.5% PH₃ (diluted with an H₂ gas) was set to 170 sccm; thepressure was set to 170 Pa; the substrate temperature was set to 280°C.; and plasma discharge was performed using the RF power sourcefrequency of 13.56 MHz and the power of the RF power source of 60 W.

FIG. 19 shows results of an analysis by secondary ion mass spectrometryof the sample. The horizontal axis represents the depth from the surfaceof the sample. The vertical axis on the left side represents theconcentrations of hydrogen, carbon, nitrogen, oxygen, and fluorine. Thevertical axis on the right side represents the secondary ion intensityof silicon. A depressed portion at a depth of approximately 45 nm to 50nm in the secondary ion intensity of silicon is an interface between thesecond semiconductor layer 407 and the impurity semiconductor layer. Apeak portion at a depth of approximately 240 nm to 245 nm in thesecondary ion intensity of silicon is an interface between the gateinsulating layer 404 and the first semiconductor layer 406. Here, thethickness of the first semiconductor layer 406 is approximately 30 nm;therefore, an interface between the first semiconductor layer 406 andthe second semiconductor layer 407 is estimated at a depth ofapproximately 210 nm to 215 nm.

In this example, the nitrogen concentration sharply decreases at theinterface between the gate insulating layer 404 and the firstsemiconductor layer 406 and then gradually increases toward the secondsemiconductor layer 407. That is, the nitrogen concentration reaches theminimum value in the first semiconductor layer 406 and keepssubstantially constant in the second semiconductor layer 407. The oxygenconcentration sharply increases at the interface between the gateinsulating layer 404 and the first semiconductor layer 406 and soondecreases. The oxygen concentration gradually decreases toward thesecond semiconductor layer 407 and keeps substantially constant in thesecond semiconductor layer 407.

The nitrogen concentration at the interface between the gate insulatinglayer 404 and the first semiconductor layer 406 (that is, the nitrogenconcentration before rapid decrease) is 8×10¹⁹ atoms/cm³. The minimumvalue of the nitrogen concentration in the first semiconductor layer 406is 2×10¹⁹ atoms/cm³. The nitrogen concentration in the secondsemiconductor layer 407 is 6×10¹⁹ atoms/cm³.

The oxygen concentration at the interface between the gate insulatinglayer 404 and the first semiconductor layer 406 is 1×10²¹ atoms/cm³. Theoxygen concentration in the second semiconductor layer 407 is lower thana lower limit of the detection.

As described above, when the gate insulating layer 404 is formed usingthe silicon nitride layer and the silicon oxide layer which is formed byoxidizing the surface of the silicon nitride layer, the peak of theoxygen concentration lies at the interface between the gate insulatinglayer 404 and the first semiconductor layer 406 and the minimum value ofthe nitrogen concentration lies in the first semiconductor layer 406.The nitrogen concentration is substantially constant in the secondsemiconductor layer 407.

Example 3

In this example, cross-sectional STEM (scanning transmission electronmicroscopy) images of samples which are each formed by stacking the gateinsulating layer and the first semiconductor layer are shown in FIGS.20A to 20C.

A process for forming samples 1 to 3 is described.

First, a silicon nitride layer with a thickness of 100 nm was formedover a glass substrate 1001.

As the glass substrate, a glass substrate with a thickness of 0.7 mm(EAGLE2000, manufactured by Corning, Inc.) was used.

The silicon nitride layer was formed under the similar conditions tothose for forming the silicon nitride layer described in Example 2.

Then, oxidation treatment was performed on the surface of the siliconnitride layer.

Similarly to Example 2, the oxidation treatment of the sample 1 wasperformed in such a manner that the surface of the silicon nitride layerwas exposed to air for approximately 5 minutes after the substrate wastransferred from the treatment chamber to the load lock chamber. Thus,the silicon oxide layer was formed on the silicon nitride layer; andthus, a gate insulating layer 1002 was formed.

The oxidation treatment of the sample 2 was performed in such a mannerthat the surface of the silicon nitride layer was exposed to an oxygenplasma atmosphere for approximately 60 seconds in the treatment chamberwhere the silicon nitride layer had been formed. Thus, the silicon oxidelayer was formed on the silicon nitride layer.

As a comparative example, the sample 3 was not subjected to oxidationtreatment on the surface of the silicon nitride layer.

Next, the first semiconductor layer was formed.

As for the sample 1, similarly to Example 2, the treatment chamber wascleaned, a protective layer was formed on the treatment chamber, and thesubstrate was transferred to the treatment chamber; thus, amicrocrystalline silicon layer with a thickness of 30 nm was formed as afirst semiconductor layer 1003 under similar conditions to those of thefirst semiconductor layer of Example 2.

As for the sample 2, in the treatment chamber where the oxidationtreatment was performed, a microcrystalline silicon layer with athickness of 30 nm was formed as a first semiconductor layer 1013 undersimilar conditions to those of the first semiconductor layer of Example2.

As for the sample 3, after the substrate was transferred from thetreatment chamber to the load lock chamber which was held in a vacuum,the treatment chamber was cleaned in a similar manner to that in Example2, and a protective film was formed on an inner wall of the treatmentchamber. Next, the substrate was transferred from the load lock chamberto the treatment chamber, and a microcrystalline silicon layer with athickness of 30 nm was formed as a first semiconductor layer 1023 undersimilar conditions to those of the first semiconductor layer of Example2.

Each of FIGS. 20A to 20C is a cross section of the samples 1 to 3.

Crystal grain boundaries are observed at the interface between the firstsemiconductor layer 1003 and the gate insulating layer 1002 in the STEMimage in FIG. 20A and at the interface between the first semiconductorlayer 1013 and the gate insulating layer 1002 in the STEM image in FIG.20B. Therefore, it is found that crystals can grow from the interfacebetween the gate insulating layer 1002 and the first semiconductor layer1003 or the interface between the gate insulating layer 1002 and thefirst semiconductor layer 1013 when a silicon oxide layer is formed onthe outermost surface of the gate insulating layer 1002.

On the other hand, in the STEM image in FIG. 20C, a crystal grainboundary is not observed at the interface between the firstsemiconductor layer 1023 and the gate insulating layer 1002 and in thevicinity thereof. Accordingly, in the case where the outermost surfaceof the gate insulating layer 1002 is a silicon nitride layer, it isfound that amorphous silicon is deposited at an early stage of thedeposition of the first semiconductor layer 1023.

Example 4

In this example, effects that a base layer has on crystallinity of amicrocrystalline semiconductor layer which is formed as a firstsemiconductor layer are described with reference to calculation results.Here, a microcrystalline silicon layer is used as a typical example ofthe microcrystalline semiconductor layer.

In this example, a crystallization process of Si in the case where animpurity element (an N atom or an O atom) was included was calculatedusing classical molecular dynamics simulation. An empirical potentialwhich characterizes the interaction between atoms is defined in theclassical molecular dynamics simulation, so that force that acts on eachatom is evaluated. A law of classical mechanics is applied to each atomand Newton's equation of motion is numerically solved, whereby motion(time-dependent change) of each atom can be deterministically tracked.

Here, in order to investigate the crystal growth of Si after a crystalnucleus of Si is generated in an a-Si layer, calculation models weremade for the case where the a-Si layer does not include any impurityelement and the case where the a-Si layer includes an impurity element(an N atom or an O atom) (see FIGS. 21A to 21C).

FIG. 21A illustrates a model in which a crystal nucleus 1111 isgenerated in an a-Si layer 1110 which does not include an impurityelement and single crystal silicon with a plane orientation (100) growsfrom the crystal nucleus 1111. Note that the a-Si layer 1110 includes aSi atom 1113.

FIG. 21B illustrates a model in which the crystal nucleus 1111 isgenerated in the a-Si layer 1110 which includes N atoms 1115 as animpurity element at 0.5 atomic %, that is, at a concentration ofapproximately 2.5×10²⁰ atoms/cm³ and single crystal silicon with a planeorientation (100) grows from the crystal nucleus 1111.

FIG. 21C illustrates a model in which the crystal nucleus 1111 isgenerated in the a-Si layer 1110 which includes O atoms 1117 as animpurity element at 0.5 atomic %, that is, at a concentration ofapproximately 2.5×10²⁰ atoms/cm³ and single crystal silicon with a planeorientation (100) grows from the crystal nucleus 1111.

Classical molecular dynamics simulation was performed at 1025° C. on theabove three calculation models illustrated in FIGS. 21A to 21C.

FIGS. 22A to 22C illustrate calculation results of change of thestructure of FIG. 21A. FIG. 22A shows a model at 0 seconds. FIG. 22Bshows a model after 0.5 nanoseconds. FIG. 22C shows a model after 1nanosecond.

FIGS. 23A to 23C illustrate calculation results of change of thestructure of FIG. 21B. FIG. 23A shows a model at 0 seconds. FIG. 23Bshows a model after 1 nanosecond. FIG. 23C shows a model after 2nanoseconds.

FIGS. 24A to 24C illustrate calculation results of change of thestructure of FIG. 21C. FIG. 24A shows a model at 0 seconds. FIG. 24Bshows a model after 0.5 nanoseconds. FIG. 24C shows a model after 1nanosecond.

Table 1 shows crystal growth rates of Si in each calculation model.

TABLE 1 Impurity element Crystal growth rate (nm/ns) {circle around (1)}None 1.1 {circle around (2)} Nitrogen 0.21 {circle around (3)} Oxygen0.80

The crystal nucleus 1111 in FIG. 22A spreads to a crystal region 1121 aof single crystal silicon in FIG. 22B and to a crystal region 1121 b ofsingle crystal silicon in FIG. 22C. Thus, it is found that the Si atom1113 grows to be a crystal in the case where the a-Si layer does notinclude an impurity element.

In the case where the a-Si layer includes an N atom, the crystal nucleus1111 in FIG. 23A spreads to a crystal region 1123 a of single crystalsilicon in FIG. 23B and to a crystal region 1123 b of single crystalsilicon in FIG. 23C. However, it is found that the crystal region issmaller and the crystal growth rate is slower as compared to the caseillustrated in FIGS. 22A to 22C, where an impurity element is notincluded.

Note that as illustrated in FIGS. 23B and 23C, the crystal growth isinhibited due to the N atom 1115 in the a-Si layer and the N atom 1115is not incorporated in the crystal region 1123 a and the crystal region1123 b of the single crystal silicon but exists in the vicinity of thecrystal grain boundary.

In the case where the a-Si layer includes the O atom 1117, the crystalnucleus 1111 in FIG. 24A spreads to a crystal region 1125 a of singlecrystal silicon in FIG. 24B and to a crystal region 1125 b of singlecrystal silicon in FIG. 24C. However, the crystal region is smaller andthe crystal growth rate is slower as compared to the case illustrated inFIGS. 22A to 22C, where an impurity element is not included. As comparedto the case illustrated in FIGS. 23A to 23C, where an N atom isincluded, the crystal region is larger and the crystal growth rate isfaster. In addition, as illustrated in FIG. 24C, it is found that the Oatom 1117 is incorporated in the crystal region 1125 b of single crystalsilicon and the crystallinity of the entire film is relatively good.Accordingly, it is considered that crystallinity of Si is not greatlyaffected when the oxygen concentration in the film is high to someextent but crystallinity of Si decreases when the nitrogen concentrationis high.

Accordingly, when a silicon oxide layer is formed as a base layer of amicrocrystalline semiconductor layer, crystallinity and a crystal growthrate of the microcrystalline semiconductor layer can be increased.

This application is based on Japanese Patent Application serial no.2009-152370 filed with Japan Patent Office on Jun. 26, 2009, the entirecontents of which are hereby incorporated by reference.

1. A thin film transistor comprising: a gate electrode; a gateinsulating layer over the gate electrode; a semiconductor layerincluding a microcrystalline semiconductor layer over the gateinsulating layer; and a source region and a drain region in contact withthe semiconductor layer, wherein a nitrogen concentration at aninterface between the gate insulating layer and the microcrystallinesemiconductor layer is higher than or equal to 5×10¹⁹ atoms/cm³ andlower than or equal to 1×10²² atoms/cm³, and wherein the nitrogenconcentration in the microcrystalline semiconductor layer reaches aminimum value which is lower than or equal to 3×10¹⁹ atoms/cm³.
 2. Thethin film transistor according to claim 1, wherein the nitrogenconcentration in the microcrystalline semiconductor layer reaches theminimum value which is higher than or equal to 1×10¹⁷ atoms/cm³.
 3. Thethin film transistor according to claim 1, wherein the thin filmtransistor is incorporated in one selected from the group consisting ofan electronic book reader, a digital photo frame, a television set, anda mobile phone.
 4. A thin film transistor comprising: a gate electrode;a gate insulating layer over the gate electrode; a first semiconductorlayer including a microcrystalline semiconductor layer over the gateinsulating layer; a second semiconductor layer including a conical orpyramidal crystal region and an amorphous semiconductor region over thefirst semiconductor layer; and a source region and a drain region incontact with the second semiconductor layer, wherein a nitrogenconcentration at an interface between the gate insulating layer and thefirst semiconductor layer is higher than or equal to 5×10¹⁹ atoms/cm³and lower than or equal to 1×10²² atoms/cm³, wherein the nitrogenconcentration in the first semiconductor layer reaches a minimum valuewhich is lower than or equal to 3×10¹⁹ atoms/cm³, wherein the nitrogenconcentration in the second semiconductor layer is higher than or equalto 1×10¹⁹ atoms/cm³ and lower than or equal to 1×10²¹ atoms/cm³, whereinan oxygen concentration at the interface between the gate insulatinglayer and the first semiconductor layer is higher than or equal to1×10¹⁹ atoms/cm³ and lower than or equal to 1×10²² atoms/cm³, andwherein the oxygen concentration in the second semiconductor layer islower than or equal to 1×10¹⁸ atoms/cm³.
 5. The thin film transistoraccording to claim 4, wherein the nitrogen concentration in the firstsemiconductor layer reaches the minimum value which is higher than orequal to 1×10¹⁷ atoms/cm³.
 6. The thin film transistor according toclaim 4, wherein the second semiconductor layer including the conical orpyramidal crystal region and the amorphous semiconductor region includesan NH group or an NH, group.
 7. The thin film transistor according toclaim 4, wherein the thin film transistor is incorporated in oneselected from the group consisting of an electronic book reader, adigital photo frame, a television set, and a mobile phone.
 8. A methodfor manufacturing a thin film transistor, comprising: forming a gateelectrode; forming a gate insulating layer using silicon nitride, overthe gate electrode; oxidizing a surface of the gate insulating layer;forming a semiconductor layer including a microcrystalline semiconductorlayer over the gate insulating layer; and forming a source region and adrain region in contact with the semiconductor layer.
 9. The method formanufacturing a thin film transistor according to claim 8, wherein theoxidizing step is performed by plasma treatment using a gas containingoxygen.
 10. The method for manufacturing a thin film transistoraccording to claim 8, wherein the thin film transistor is incorporatedin one selected from the group consisting of an electronic book reader,a digital photo frame, a television set, and a mobile phone.